System for distributing and controlling color reproduction at multiple sites

ABSTRACT

In the color imaging system, multiple rendering devices are provided at different nodes along a network. Each rendering device has a color measurement instrument for calibrating the color presented by the rendering device. A rendering device may represent a color display in which a member surrounds the outer periphery of the screen of the display and a color measuring instrument is coupled to the first member. The color measuring instrument includes a sensor spaced from the screen at an angle with respect to the screen for receiving light from an area of the screen. A rendering device may be a printer in which the measuring of color samples on a sheet rendered by the printer is provided by a sensor coupled to a transport mechanism which moves the sensor and sheet relative to each other, where the sensor provides light from the sample to a spectrograph. The color measuring instruments provide for non-contact measurements of color samples either displayed on a color display, or printed on a sheet, and are self-calibrating by the use of calibration references in the instrument.

This application claims the benefit of U.S. Provisional PatentApplication Ser. No. 60/056,947, filed Aug. 25, 1997, and is related toco-pending patent application Ser. No. 08/606,883, filed Feb. 26, 1996.

FIELD OF THE INVENTION

The present invention relates to a system (method and apparatus) fordistributing and controlling color reproduction at multiple sites, andparticularly to, a system for distributing and controlling the coloroutput of rendering devices, such as color monitors, proofing devices,and presses, at multiple sites or nodes of a network to provide auniform appearance of color within the output colors attainable at eachrendering device. The invention utilizes a color measurement instrumentassociated with each rendering device for obtaining color calibrationdata for calibrating the rendering device. The system is controlled bycomputers at each node and utilizes a data structure, referred to hereinas a Virtual Proof, to store and distribute color transformationinformation in the network. Color image data representing one or morepages or page constituents can be distributed to the nodes separatelyfrom the Virtual Proof.

BACKGROUND OF THE INVENTION

In the printing and publishing industry, the increasing modularity ofmanufacturing operations is enabling customization of products. At thesame time, pressures to reduce inventories and to keep them fresh aredriving a trend toward just-in-time production and stocking. Whereverthe manufacturing can be decentralized and distributed geographically,just-in-time production is facilitated because producers are closer toconsumers in space and time. There is an ecological dividend, as well,in reduced demands on the transportation system. Overall product costmay decrease with shipping expense. At the same time, however, thechallenge of maintaining uniform quality across a network of productionsites increases. Minimizing startup waste gains in importance as doescompensating for uneven skill and experience of operators. Color is akey variable to control because it affects product appearance andperceived quality.

Today for example, a magazine with a national circulation of 5 millionmay be printed at 5 regional plants scattered across the nation.Distribution (transportation and postage) generally account for onethird of the cost of the product while transit time has a significantimpact on product “freshness,” i.e., the timeliness of the informationdelivered.

Production is as centralized as it is partly in order to maintainreasonably homogeneous quality. Nevertheless, printed color varieswithin a press run and from site to site because there have been onlylimited means of coordinating control of product appearance among sites.The scope and significance of this problem is apparent when oneconsiders how much commerce and economic activity are leveraged byadvertising and that generally more than 60% of all printing isadvertising-related. Analogous problems also arise in other media,particularly now that digital video images can be edited in real timeand broadcast directly.

The preceding paragraphs have spoken about parallel mass-production atmultiple sites. Publishing is also distributed in the sense that thesequential steps of preparation for volume production occur at distinctsites, as illustrated in FIG. 1. Oftentimes, the sites representdifferent business entities (for example, an advertising agency, apublisher, or an engraver) which are geographically separated. Solidlines in FIG. 1 represent links connecting the sites in the productionprocess. Overlaid in FIG. 1 are dotted boundaries indicating a clusterof pre-publishing facilities which handle sequential phases of theprocess under Product Prototype 1, and printing facilities which may beinvolved in parallel Volume Production 2.

Currently prevalent volume printing technologies such as offsetlithography, employ a printing “plate” which bears fixed information andis the tool or die of volume production. The tool is mounted on a pressand numerous copies of the printed product are stamped out. Fortechnologies such as ink jet and electrophotography the information onthe plate can be changed from one revolution of the press to the next.This technological development enables significant product customizationand is compatible with just-in-time production scenarios. It alsoenables process control in which the electronic data flowing to thedevice are modified to adapt to changes in the marking engine. However,the consistency (or repeatability) of these processes makes them evenmore susceptible to regional variations in quality across the productionsites than lithography and its relatives.

For all of the printing technologies mentioned, there is a commonproblem of uniform and accurate color reproduction. Analogous problemsalso exist in other media for distributing color graphic or imagecontent, such as CDROM or the Internet. Consider an advertiser in NewYork, physically removed from the five production sites mentioned above,or the more numerous sites that may be involved in the future. There isa keen interest in having the product portrayed in as faithful an accordwith the advertiser's artistic conceptions as possible, even when the adis to appear in different publications printed on different substratesby different machinery or in the same publication disseminated throughdifferent media.

Today, the approval cycle, the means by which print buyer and printerreach contractual agreement about the acceptability of product, oftenproceeds as outlined in FIG. 1 in the publication segment of theindustry. Phases or functions of production are enclosed in ellipses 1a. 1 b and 1 c and key products of theses functions are enclosed byrectangles 3, 5, 6, 7, 8 and 9. The dashed line between creation 1 a andprepress 1 b shows the blurring of those functions in the development ofintermediate products, such as page constituents like lineart, images,text and comps. Prepress 1 b on the way to film 5 may includerasterization, separation and screening 4. However, acceptance ofcomputer-to-plate technology will blur the boundary between prepress 1 band production 1 c.

The long, heavy boundary line between press-proofing in low volumereproduction 1 c and high volume production 2 represent the distinctnessof the two functions; the former is carried out by engravers orcommercial printers. Note that volume production 2 may occur at multiplesites. Linkages in the approval process are shown by arcs 10 a and 10 bat the bottom of FIG. 2, where 10 a is the traditional off-press proofand 10 b is a press proof. The transactions in the approval processinvolve one or more generations of static proofs which are prepared withlimited regard for the capabilities of the final, volume-productiondevices. In other words, there is no feedback from production to earlierfunctions. The process results in idle time for equipment and personneland waste of consumables (paper, ink etc.) Furthermore, it usually doesnot give the print buyer any direct say about the appearance of theultimate product unless the buyer travels to the printing plant, anexpensive proposition.

The workflow for commercial printing is slightly different from thatdescribed above, since press-proofs are seldom used and the print buyeror his agent often go to the printer's for approval. However, theessential lack of feedback is also prevalent in the commercialenvironment as well.

It is clear that a common language of color could insure improvedcommunication, control and quality throughout the sites of FIG. 1. Thecommon language is a color space, typically based on the internationallyaccepted Standard Observer which quantifies color in terms of whatnormal humans see, rather than in terms of specific samples or instancesof color produced by particular equipment. The Standard Observer is thebasis of device-independent, colorimetric methods of image reproductionand is defined by the Commission Internationale de L'Eclairage in CIEPublication 15.2, 1986. Central Bureau of the CIE. Box 169. Vienna,Austria. Approximately uniform perceptual color spaces based upon theStandard Observer are also discussed in this publication.

Color Space is defined as a three-dimensional, numerical scheme in whicheach and every humanly perceivable color has a unique coordinate. Forexample, CIELAB is a color space defined by the CIE in 1976 to simulatevarious aspects of human visual performance. Color in the next sectionwill refer to CIE color or what we see, while colorant will refer toparticular physical agents, such as dyes, pigments, phosphors, and thelike that are instrumental in producing sensations and perceptions ofcolor in a human at rendering devices, such as presses and videoscreens.

An early machine for converting color image data to colorantspecifications for a 3 or 4-channel reflection reproduction process wasdescribed by Hardy and Wurzburg (Color correction in color printing. J.Opt. Soc. Amer., Vol. 38, pp. 300-307, 1948.) They described anelectronic network provided with feedback to control convergence to thesolution of an inverse model of colorant mixture and produce 4-colorantreproductions indistinguishable from 3-colorant reproductions made underlike conditions. The set point for the control was the color of theoriginal. This work serves as a starting point for many subsequentdevelopments in the art particularly as regards device independent colorreproduction technologies and color separation, i.e., the preparation ofprinting plates for 3 or more colorants.

In U.S. Pat. No. 2,790,844, Neugebauer discloses a system to extend theHardy-Wurzburg machine. It describes the capture and representation ofcolor imagery in a colorimetric (or device independent) coordinatesystem. To enable an operator to judge the effect of color correctionswhile he is making these color corrections, the system provides for asoft proof realized by projecting video images onto the type of paperstock to be used in the final reproduction with careful regard to makingthe surround illumination and viewing conditions comparable to thoseprevailing when the final product is viewed. The objective of the softproof was to simulate a hard copy proof or final print. This is incontrast to U.S. Pat. No. 4,500,919, issued to Schreiber, whichdiscloses a system to match the hard copy to the monitor image.

Concerning models of color formation by combination of colorants.Pobboraysky (A proposed engineering approach to color reproduction. TAGAProceedings, pp. 127-165, 1962) first demonstrated the use of regressiontechniques (curve fitting) to define mathematical relationships betweendevice independent color (in the CIE sense) and amounts of colorant withaccurate results. The mathematical relationships took the form of loworder polynomials in several variables.

Schwartz et al. (Measurements of Gray Component Reduction in neutralsand saturated colors, TAGA Proceedings, pp. 16-27, 1985) described astrategy for inverting forward models (mathematical functions forconverting colorant mixtures to color.) The algorithm was similar toHardy and Wurzburg's but implemented with digital computers; it consistsof iteratively computing (or measuring) the color of a mixture ofcolorants, comparing the color to that desired and modifying thecolorants in directions dictated by the gradients of colorants withrespect to color error until color error is satisfactorily small. Colorerror is computed in CIE uniform coordinates. The context of the workwas an implementation of an aspect of the art known as Gray ComponentReplacement (GCR.)

Because normal human color perception is inherently 3-dimensional, theuse of more than 3 colorants is likely to involve at least one colorantwhose effects can be simulated by a mixture of two or more of the others(primaries.) For example, various amounts of black ink can be matched byspecific mixtures of the primary subtractive colorants cyan, magenta andyellow. The goal of Schwartz et al. was a method for findingcolorimetrically equivalent (indistinguishable in Hardy and Wurzburg'swords) 4-colorant solutions to the problem of printing a given colorthat used varying amounts of black. Additional colorants (more than 3)are used to expand the gamut; black enables achievement of densities inreflection reproduction processes that are not otherwise available. Agamut is the subset of human perceivable colors that may be outputted bya rendering device. However, increased reliance on black through GCR hasother important dividends: a) there is an economic benefit to theprinter and an environmental benefit at large in using less colored ink,b) use of more black affords better control of the process.

Boll reported work on separating color for more than four colorants (Acolor to colorant transformation for a seven ink process. SPIE Vol.2170, pp. 108-118, 1994, The Society for Photo-Optical andInstrumentation Engineers. Bellingham. Wash.). He describes theSupergamut for all seven colorants as a union of subgamuts formed bycombinations of subsets of 4-at-a-time of the colorants. Because of themanner in which his subsets are modeled, the method severely limitsflexibility in performing GCR.

Descriptions of gamuts in colorimetric terms date at least to Neugebauer(The colorimetric effect of the selection of printing inks andphotographic filters on the quality of multicolor reproductions, TAGAProceedings, pp. 15-28, 1956.) The first descriptions in the coordinatesof one of the CIE's uniform color spaces are due to Gordon et al. (Onthe rendition of unprintable colors, TAGA Proceedings, pp. 186-195,1987.) who extended the work to the first analysis of explicit gamutoperators—i.e., functions which map colors from an input gamut tocorrespondents in an output gamut.

A detailed review of requirements of and strategies for color systemscalibration and control was published by Holub, et al. (Color systemscalibration for Graphic Arts, Parts I and II, Input and output devices,J. Imag. Technol., Vol. 14, pp. 47-60, 1988.) These papers cover fourareas: a) the application of color measurement instrumentation to thecalibration of devices, b) requirements for colorimetrically accurateimage capture (imaging colorimetry,) c) development of renderingtransformations for 4-colorant devices and d) requirements for softproofing.

Concerning the first area (a), U.S. Pat. No. 5,272,518, issued toVincent, discloses a portable spectral colorimeter for performingsystem-wide calibrations. The main departure from the work of Holub etal., just cited, is in the specification of a relatively low cost designbased on a linearly variable spectral filter interposed between theobject of measurement and a linear sensor array. Vincent also mentionsapplicability to insuring consistent color across a network, but doesnot discuss how distributed calibration would be implemented. There isno provision for self-checking of calibration by Vincent's instrumentnor provision for verification of calibration in its application.

U.S. Pat. No. 5,107,332, issued to Chan, and U.S. Pat. No. 5,185,673,issued to Sobol, disclose similar systems for performing closed-loopcontrol of digital printers. Both Chan and Sobol share the followingfeatures: 1) They are oriented toward relatively low quality, desktopdevices, such as ink jet printers. 2) An important component in eachsystem is a scanner, in particular, a flat-bed image digitizer. 3) Thescanner and printing assembly are used as part of a closed system ofcalibration. A standardized calibration form made by the printing systemis scanned and distortions or deviations from the expected color valuesare used to generate correction coefficients used to improve renderings.Colorimetric calibration of the scanner or print path to a deviceindependent criterion in support of sharing of color data or proofing onexternal devices was not an objective. 4) No requirements are placedupon the spectral sensitivities of the scanner's RGB channelsensitivities. This has ramifications for the viability of the methodfor sets of rendering colorants other than those used in the closedprinting system, as explained below.

In Sobol, the color reproduction of the device is not modeled; ratherthe distortions are measured and used to drive compensatory changes inthe actual image data, prior to rendering. In Chan, there appears to bea model of the device which is modified by feedback to controlrendering. However, colorimetric calibration for the purposes ofbuilding gamut descriptions in support of proofing relationships amongdevices is not disclosed.

Pertaining to item (b) of the Holub. et al. paper in J. ImagingTechnology and to the foregoing patents, two articles aresignificant: 1) Gordon and Holub (On the use of linear transformationsfor scanner calibration, Color Research and Application. Vol. 18, pp.218-219, 1993) and 2) Holub (Colorimetric aspects of image capture,IS&T's 48th Annual Conference Proceedings, The Society for ImagingScience and Technology, Arlington. Va., pp. 449-451, May 1995.) Takentogether, these articles demonstrate that, except when the spectralsensitivities of the sensor's channels are linear combinations of thespectral sensitivity functions of the three human receptors, the gamutof an artificial sensor will not be identical to that of a normal human.In other words, the artificial sensor will be unable to distinguishcolors that a human can distinguish. Another consequence is that thereis generally no exact or simple mathematical transformation for mappingfrom sensor responses to human responses, as there is when the linearitycriterion set forth in this paragraph is satisfied by the artificialsensor.

To summarize the preceding paragraphs: The objective of measuring thecolors of reproduction for the purpose of controlling them to a humanperceptual criterion across a network of devices in which proofing andthe negotiation of approval are goals is best served when the imagesensors are linear in the manner noted above.

Results of a colorimetric calibration of several printing presses werereported by Holub and Kearsley (Color to colorant conversions in acolorimetric separation system. SPIE Vol. 1184. Neugebauer MemorialSeminar on Color Reproduction, pp. 24-35, 1989.) The purpose of theprocedure was to enable workers upstream in the production process in aparticular plant to be able to view images on video display devices,which images appeared substantially as they would in production,consistent with the goals of Neugebauer in U.S. Pat. No. 2,790,844.Productivity was enhanced when design could be performed with awarenessof the limitations of the production equipment. The problem was that theproduction equipment changed with time (even within a production cycle)so that static calibration proved inadequate.

In U.S. Pat. No. 5,182,721, Kipphan et al. disclose a system for takingprinted sheets and scanning specialized color bars at the margin of thesheets with a spectral colorimeter. Readings in CIELAB are compared toaim values and the color errors so generated converted into correctionsin ink density specifications. The correction signals are passed to theink preset control panel and processed by the circuits which control theinking keys of the offset press. Operator override is possible and isnecessary when the colorimeter goes out of calibration, since it is notcapable of calibration self-check. Although the unit generates datauseable for statistical process control, the operator must be pro-activein sampling the press run with sufficient regularity and awareness ofprinted sheet count in order to exploit the capability. The process isclosed loop, but off-line and does not read image area of the printedpage. Important information regarding color deviations within the imagearea of the press sheet is lost by focussing on the color bars.

On page 5 of a periodical Komori World News are capsule descriptions ofthe Print Density Control System, which resembles the subject of Kipphanet al. Also described is the Print Quality Assessment System, whichpoises cameras over the press. The latter is primarily oriented towarddefect inspection and not toward on-line color monitoring and control.

Sodergard et al. and others (On-line control of the colour print qualityguided by the digital page description, proceedings of the 22ndInternational Conference of Printing Research Institutes. Munich.Germany. 1993 and A system for inspecting colour printing quality. TAGAProceedings. 1995) describe a system for grabbing frames from the imagearea on a moving web for the purposes of controlling color, controllingregistration and detecting defects. The application is in newspaperpublishing. Stroboscopic illumination is employed to freeze frames ofsmall areas of the printed page which are imaged with a CCD camera. Thedrawback of the Sodergard et al. system is that color control lacks thenecessary precision for high quality color reproduction.

Optical low pass filtering (descreening) technology relevant to thedesign of area sensors for imaging colorimetry is discussed in U.S. Pat.No. 4,987,496, issued to Greivenkamp, and Color dependent opticalprefilter for the suppression of aliasing artifacts, Applied Optics.Vol. 29, pp. 676-684, 1990.)

Paul Shnitser (Spectrally adaptive acousto-optic tunable filter for fastimaging colorimetry, Abstract of Successful Phase I Proposal to U.S.Dept. of Commerce Small Business Innovation Research Program. 1995) andClifford Hoyt (Toward higher res, lower cost quality color andmultispectral imaging, Advanced Imaging, April 1995) have discussed theapplicability of electronically tunable optical/spectral filters tocolorimetric imaging.

In Thin-film measurements using SpectraCube™, (Application Note for ThinFilm Measurements. SD Spectral Diagnostics Inc., Agoura Hills; CA91301-4526) Garini describes a spectral imaging system employing “ . . .a proprietary optical method based on proven Fourier spectroscopy, whichenables the measurement of the complete visible light spectrum at eachpixel . . . ”

The applicability of neural network (and other highly parallel andhybrid) technologies to the calibration and control of rendering deviceshas been considered by Holub (“The future of parallel, analog and neuralcomputing architectures in the Graphic Arts.” TAGA Proceedings, pp.80-112, 1988) and U.S. Pat. No. 5,200,816, issued to Rose, concerningcolor conversion by neural nets.

A formalism from finite element analysis is described in Gallagher.“Finite element analysis: Fundamentals.” Englewood Cliffs. N.J.,Prentice Hall, pp. 229-240, 1975, for use in the rapid evaluation ofcolor transformations by interpolation.

Area (d) of the earlier discussion of Holub et al.'s review referred toprinciples guiding the design and application of softproofing: methodsof calibrating video displays, evaluation of and compensation forillumination and viewing conditions, representation of how imagery willlook on client devices and psychophysical considerations of matchingappearance across media.

In the article “A general teleproofing system.” (TAGA Proceedings, 1991,The Technical Association of the Graphic Arts, Rochester, N.Y.)Sodergard et al. and others discuss a method for digitizing the analogimage of an arbitrary monitor for transmission through an ISDNtelecommunications link to a remote video display. The method involvesthe transmission of the actual image data, albeit at the relatively lowresolution afforded by the frame buffers typical of most displays. Thismethod lacks any provision for calibration or verification of thedevices at either end of a link and also lacks the data structuresneeded to support remote proofing and negotiation of color approval.

In U.S. Pat. No. 5,231,481, Eouzan et al. disclose a system forcontrolling a projection video display based on cathode ray tubetechnology. A camera is used for capturing image area of a display. Theprocedures are suited to the environment in which the displays aremanufactured and not to where they are used. Concepts of colorimetriccalibration of the display and control of display output to acolorimetric criterion are not disclosed.

In U.S. Pat. No. 5,309,257, Bonino et al. disclose a method forharmonizing the output of color devices, primarily video displaymonitors. In a first step, measurements of the voltage in vs. luminanceout relationship are made for each of the three display channelsseparately and then the V/L functions of all the devices are adjusted tohave a commonly achievable maximum. This is assumed to insure that alldevices are operating within the same gamut—an assumption which is onlytrue if the chromaticities of the primaries in all devices aresubstantially the same. The single-channel luminance meter (aphotometer) described as part of the preferred embodiment does notpermit verification of the assumption. Bonino et al. thus employsphotometric characterization of devices and lacks a colorimetriccharacterization.

The Metric Color Tag (MCT) Specification (Rev 1.1d. 1993. Electronicsfor Imaging. Inc. San Mateo, Calif. is a definition of data required indata files to allow color management systems to apply accurate colortransformations. The MCT thus does not provide a file format definingthe full specification of color transformations in the context ofdistributed production and color-critical remote proofing.

In contrast to the MCT, the International Color Consortium (ICC) ProfileFormat is a file format, and is described in the paper, InternationalColor Consortium Profile Format (version 3.01, May 8, 1995). A profileis a data table which is used for color conversion—the translation ofcolor image data from one color or colorant coordinate system toanother. The ICC Profile Format provides for embedding profiles withimage data. This generates large data transfers over a network wheneverprofiles are updated. Further, the ICC Profile. Representation ofdevices in the ICC Profile Format is limited in supporting “scnr”(scanner). “mntr” (video display monitor) and “prtr” (printer) devicetypes, and is not readily extendable to other types of devices.

Interactive remote viewing is described for imagexpo applicationsoftware from Group Logic, Inc., in the article “Introducing imagexpo1.2: Interactive remote viewing and annotation software for the graphicarts professional” and “Before your very eyes.” (reprinted fromPublishing & Production Executive, August 1995), which acknowledges thatextant tools do not enable remote handling of color-critical aspects ofproofing.

Color management refers to the process of converting digital image datafrom a format or representation suited for one device to one suited foranother. Often, the conversion employs a device independent intermediarycolor space such as one promulgated by the Commission Internationale deL'Eclairage (CIE.) A device-independent color space provides a means ofquantifying colors as a color-normal human perceives them (or, moreprecisely, matches them) rather than as particular samples or instancesof color produced by a device.

For example, image data may be introduced to a computer system byscanning. The data are initially in a coordinate system which isspecific to the scanner and not understandable by any other device. Inorder to reproduce the image with a printer so that a human recognizesthe print as a faithful replica of the original image, it is necessaryto translate scanner codes to printer codes.

Color translation may be performed by an expert human knowledgeable inthe languages of the two devices. This is the traditional method ofcolor management. Alternatively, both devices may be calibrated byinstruments which simulate human color-matching. The instruments analyzea sample to produce a set of color coordinates identical to thoseselected by the CIE Standard Observer in the original color matchingexperiments. The Standard Observer represents an average, color-normalhuman.

The calibration data acquired from a device with a color matchinginstrument are commonly used in the preparation of translators whichconvert the color coordinates of one device to those of another throughintermediate, device-independent coordinates. An important motivationfor introducing color instrumentation and color management to theworkflow is reduction of the level of skill required of the humanoperator(s.) The benefits of the automation are enlargement of themarket for color in documents and a reduction of the cost of color.

Typically, calibration devices are limited in one or more of thefollowing ways. First, many of the devices require manual measurementsof samples under circumstances conducive to operator error. An unskilledoperator is ill-equipped to recognize likely problems in the data.Second, an instrument may require physical contact with the copy andconsequent scuffing or transfer of fingerprints and skin oils. Samplesare routinely affected by this before they are measured and the accuracyof a dataset is compromised. Instruments used with monitors are affixedwith suction devices leaving rings of residue which have to be cleanedup or which affect subsequent measurements. The devices clutter theworkspace when not in use and require significant operator involvementin measurement. Third, an instrument may require calibration by theoperator. A black trap may be provided whose purpose and properapplication is not understood by an unskilled operator and whichconstitutes desktop clutter most of the time. Likewise, proper use,cleaning and maintenance of white calibration plaques often used incalibration are not usually performed. Fourth, instrument-to-instrumentvariation precludes calibration of devices at different sites to atolerance that will support confident, remote proofing. Thus, typicalcalibration instrumentation of a rendering device is not sufficientlyfool-proof to serve the intended purpose of automating the process ofinterdevice color reproduction.

SUMMARY OF THE INVENTION

It is the principal object of the present invention to provides animproved color imaging system in which color measurements are accuratelyprovided by rendering devices using a calibration system including thecomputer coupled to each rendering device and a color measurementinstrument.

Another object of the invention is to provide improved color measuringsystems, methods, and apparatuses for a rendering device, such as acolor display or printer, for enabling color calibration and VirtualProofing.

A further object of the present invention is to provide improved colormeasuring systems, methods or apparatuses for a rendering device whichare self- or auto-calibrating, minimize user involvement, and arenon-contact with the sample being measured.

Yet a further object of the present invention is to provide an improvedsystem for controlling color reproduction on network of nodes havingrendering devices, in which a computer server at one node stores adatabase of color profiles for calibrating rendering device at othernodes.

Briefly described, a system embodying the present invention forcalibrating a color display includes an assembly of a first membersurrounding the outer periphery of a display, and a color measuringinstrument coupled to the first member and spaced from the screen at anangle with respect to the screen for receiving light from the screen.The color measurement instrument comprises a housing, at least onesensor in the housing for converting light received by the sensor fromthe screen into electrical signals representative of the light, opticsin the housing for focusing light onto the sensor, and a controlcircuitry for receiving the electrical signals from the sensor andconverting the electrical signals into signals representative of thecolor or the light received by the sensor. A computer coupled to thecolor display receives the signals from the color measurement instrumentfor calibrating the display and enabling Virtual Proofing utilizing thecolor display.

Another system embodying the present invention provides for measuringcolor samples rendered by a printer includes a mechanism fortransporting a sheet rendered from the printer having color samples, andat least one optical sensor coupled to the mechanism which is directedto the sheet to measure the color of the color sample. The mechanism maybe separate from the printer or integrated in the printer. The sensorhas at least one fiber optic probe coupled to a spectrograph. Thespectrograph can automatically obtain references for checking itscalibration.

A method is also provided by the present invention for maintainingcalibration of a color display having a screen. The method comprises thesteps of adjusting the amount of light from the screen when the screenis dark to account for ambient light, neutral balancing the color of thedisplay, measuring the gamma in each color channel of the display, andadjusting the color produced by the display in accordance with thegammas measured in each color channel. This method is especially usefulfor cathode ray tube type color displays.

Apparatuses are also provided by the present invention for measuringcolor from samples rendered by a rendering device which utilizes aspectrograph and incorporates references for autocalibration of thespectrograph.

One of such apparatuses includes a dual beam spectrograph having firstand second inputs, and a light source. A first fiber optic transmitslight from the light source to illuminate the sample. A second fiberoptic transmits light from the light source to the first input of thespectrograph. A third fiber optic receives light from the sample, andtransmits the received light to the second input of the spectrograph.One or more sensors also receive light from the first fiber optic,wherein signals from the sensors provided information for checking thecalibration of the spectrograph.

Another of such apparatuses includes a light source for illuminating asample, and a one-dimensional array of fiber optics. A first fiber opticof this array receives light from said light source. A second fiberoptic of this array receives light representing a dark reference. Athird fiber optic of this array transmits light of one or more knownwavelengths, while the remaining fiber optics of this array receivelight along one-dimension from the sample. A spectrograph is providedwhich receives the light from the array of fiber optics and outputs aspectrum in accordance with the light received from the array of fiberoptics, where the part of the spectrum related to the first, second, andthird fiber optics provide information for checking the calibration ofthe spectrograph. Thus, the part of the line of light from the first,second, and third fiber optics automatically provide calibrationreferences.

The color measuring instruments incorporating the systems, method, andapparatuses described herein provide for self-calibration byincorporating calibration references into the instrument. Measurementsof the reference may be made frequently so that measurements of unknownscan be compared to them. Preferably, a reference measurement is takeneither simultaneously or successively with each reading of an unknownsample. The computer, coupled to the rendering device, outputs imagesupon the rendering device corresponding to the calibration references,and the calibration mechanism read color calibration data from theoutputted images. The color measurement instrument for a color displaymay be self-calibrating by the use of a second sensor which is protectedfrom receiving any light. In addition to self-calibration, the colormeasuring instruments described herein further improve accuracy byrequiring only a minimum of user involvement. In the case of reflectionor transmission measurements, a transport mechanism may be actuated byclick of computer mouse or, preferably, by insertion of the sheet in thetransport mechanism. Upon actuation, samples inserted in the transportmechanism are measured and the measurements processed without furtheroperator intervention. In the case of a color display, the sensor (i.e.,light-collecting optics) are located in a circumferential membersurrounding the periphery of the color display. The device is positionedand measurement scheduled unobtrusively and the user is relieved ofresponsibility other than linking the pickup to a control unit.

The color measuring instruments may be located in modules to facilitatetheir incorporation with rendering device. For example, the transportmechanism for physical copy and associated light-collecting opticsconstitute a module distinct from the module which attaches to videodisplay and from the module containing sensor(s) and controlelectronics. Light-collecting modules may be connected to the controlmodule by fiber optic links.

A further system embodying the present provides for controlling colorreproduction in which one of a network of nodes has a computer serverhaving a database which stores data for calibration rendering devices atother of the nodes. The data may represent one or more color profiles.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other features, objects, and advantages of theinvention will become more apparent from a reading of the followingdetailed description in connection with the accompanying drawings, inwhich.

FIG. 1 is a diagram of the typical sites involved in preparing volumeproduction of color products;

FIG. 2 is a diagram showing a conventional workflow for publicationprinting;

FIG. 3A shows the system in accordance with the present invention;

FIG. 3B shows a configuration of a color measuring instrument sensor fora video screen display;

FIG. 3C shows geometries of a color measuring instrument sensor formaking non-contact color measurements of reflective substrate;

FIG. 3D shows use of a color measurement instrument to estimate acomposite spectral function given knowledge of the underlying spectralfunctions of the colorants being mixed;

FIG. 4A illustrates the devices of the system separated into deviceclasses inheriting various structures and procedures for colorcalibration and transformation;

FIG. 4B is a process diagram for color transformation of a class ofdevices including linear color measuring instruments;

FIG. 4C is a process diagram for color transformation of a class ofrendering devices including video-color displays;

FIG. 5 is a process diagram for calibrating a class of rendering devicesincluding printers and presses at a node in the system of FIG. 3A toprovide color transformation information;

FIG. 6A is a flow chart detailing step 1 in FIG. 5, preparinglinearization functions;

FIG. 6B is a flow chart detailing step 2 in FIG. 5, renderingcalibration forms;

FIG. 7 is a flow chart detailing step 3 in FIG. 5, measuring calibrationforms and providing calibration data;

FIG. 8 is a flow chart detailing step 4 in FIG. 5, building a forwardmodel based on the calibration data from step 3 of FIG. 5;

FIG. 9A is a flow chart detailing step 5 in FIG. 5, preparing gamutdescriptor data for the rendering device and preparing a forward modeltable based on the forward model from step 4 in FIG. 5;

FIG. 9B is an illustration of the operators and operands evaluating thepolynomial function of the forward model in the case of two independent(colorant) variables. C and M;

FIG. 9C depicts a hypercube in the coordinates of the Cyan. Magenta.Yellow and Black colorants in which all colors producible with a CMYKprinter are contained within the hypercube;

FIG. 9D is an illustration of a data structure for interpolation in 3dimensions which may use either pre- or post-conditioning look-uptables;

FIG. 9E is a graphical illustration of linear interpolator in twodimensions;

FIG. 10A is a flow chart detailing step 6 in FIG. 5; inverting theforward model table to provide a prototype transformation table;

FIG. 10B is an example of the hypercube of FIG. 9C where the colorantvalues are transformed into device independent color coordinates;

FIG. 11 is a flow chart detailing step 7 of FIG. 5, finishing the gamutdescriptor data;

FIG. 12 is a flow chart detailing step 8 of FIG. 5, filling in anymissing entries of prototype transformation table, computing blackutilization (GCR) functions for all colors of the table having multipleblack solutions and marking unprintable colors NULL;

FIG. 13 is a flow chart detailing step 9 of FIG. 5 which includes:converting colorants of prototype transformation table based on blackcolor data; building color to color′ transform table based on gamutconfiguration data; and combining the color to color′ transformationtable and the converted prototype transformation table to provide arendering table;

FIG. 14 is an illustration of the construction of a simple gamutoperator of the gamut configuration data embodying properties ofinvertibility and reciprocality;

FIGS. 15A and 15B show the constituents of local and shareablecomponents in the data structure of the Virtual Proof;

FIG. 15C is an example of a tagged file format for the shared componentsof the Virtual Proof of FIGS. 15A and 15B;

FIG. 16A is a flow chart of the process for calibrating a renderingdevice having more than four colorants by adding non-neutral auxiliarycolorants to a rendering transformation;

FIG. 16B is a flow chart of the process for calibrating a renderingdevice having more than four colorants by adding a neutral auxiliarycolorant to a rendering transformation, in which FIGS. 16A and 16B areshown connected;

FIG. 17 is a flow chart showing the process for preparing a gamutfilter, a data structure which facilitates the comparison of gamuts oftwo or more rendering devices;

FIGS. 18A and 18B are a flow chart for virtual proofing using the systemof the present invention;

FIG. 19 is a flow chart of the verification procedures employed toestimate process variations based on measurements of color errors;

FIG. 20 is a flow chart of the process of preparing a three-dimensionalcolor histogram of color image data in providing aresolution-independent analysis of color error relative to a criterion;

FIG. 21A is a menu of the Graphical User Interface (GUI) to theapplication software to enable configuration of the network of nodes,remote conferencing and proofing and oversight of the processes involvedin calibrating devices in the systems of FIG. 3A;

FIG. 21B is a menu at the second level of hierarchy in the GUI toprovide access to tools for configuring the network connections andcommunications protocols;

FIG. 21C is a menu at the second level of hierarchy in the GUI to enablea user to manipulate the process of making color transformations at arendering device;

FIG. 21D is a menu at the second level of hierarchy in the GUI to enablea user to oversee the process of using color transformations at arendering device;

FIG. 21E is a menu at the third level of hierarchy of the GUI whichdepicts the User interface to black utilization tools in providing blackcolor data, and to neutral colorant definitions;

FIG. 21F is a menu at the third level of hierarchy of the GUI whichdepicts the User interface to gamut processing at a rendering device inthe network;

FIG. 22 is a block diagram of an example of the system of FIG. 3A;

FIG. 23 is a graph illustrating the spectrum of the green phosphoremission from a cathode ray tube (CRT) color display;

FIG. 23A is a graph illustrating the spectral emission of a red phosphoremission from two different models of colorimeters;

FIG. 24 is a block diagram of the configuration of a color measuringinstrument for a video screen display which is similar to FIG. 3A.

FIG. 24A is a perspective view showing an example of an assembly formounting a color measuring instrument to a video screen display;

FIGS. 24B, 24C and 24D are perspective view of the cowel, arm member,and one of the brackets, respectively, of FIG. 24A;

FIG. 25 is a block diagram of the configuration of a color measuringinstrument for a video screen display of FIG. 24 which includes aviewing box;

FIG. 26 is a block diagram showing the viewing box of FIG. 25 in furtherdetail;

FIG. 27 is a block diagram of part of the sensor of the colormeasurement instrument located in the cowel of the assembly of FIG. 24A;

FIG. 28 is a block diagram of the color measurement instrument with thesensor of FIG. 27 and control circuity;

FIG. 29 is table of the a command set uses by a computer to communicatewith the color measurement instrument of FIG. 28;

FIG. 30 is a graph of the gamma of the green color channel of the colormeasurement instrument of FIG. 28;

FIG. 31 is a high level flow chart showing the operation of the systemin accordance with the present invention for soft proofing to a colordisplay;

FIG. 32 is a flow chart for the process of maintaining a color displayin calibration;

FIG. 33 is a block diagram of a color measurement instrument inaccordance with the present invention utilizing a dual beamspectrograph; and

FIG. 34 is a block diagram of the a color measurement instrument inaccordance with the present invention utilizing a concentricspectrograph.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 3A, the system 100 of the present invention is shown.System 100 has a network 11 having a pipe 11 a through which multiplenodes (or sites) of network 11 can be linked for data flow betweennodes. Network 11 may be a telecommunication network, WAN, LAN (with aserver) or Internet based. Two types of nodes are present in system 100,prototype nodes 102 and production nodes 104. For purposes ofillustration, only a general node of each type is shown in FIG. 3A,however there may be multiple nodes of each type in network 11. Network11 is modifiable to be configured by any one node to connect any two ormore nodes in system 100. Each node has a micro-processor basedcomputer, with a network communication device, such as a modem, which ispart of a system having a rendering device for producing colorreproduction and color measuring instrument (CMI) for measuring thecolor output of the rendering device. The computer may be a programmablegeneral purpose computer or mainframe computer. Although a computer at anode is preferred, alternatively, the computer may be omitted at a nodeand the node operated remotely via pipe 11 a from another node.

Prototype nodes 102 allow a user to perform pre-publishing functions insystem 100, such as proofing (hard or soft), as well as the input ofdigital color image data. A user may interface with the node throughstandard interface devices, such as a keyboard or mouse. Renderingdevices in system 100 define any type of system or device for presentinga color reproduction in response to digital color signals. The renderingdevices of prototype node 102 are proofing devices, such as video screendisplay device 17 or proofer device 16. Proofing device 16 are hard copydevices, such as analog film-based devices, dye diffusion thermaltransfer devices, ink jet printers, xerographic printers, and othersimilar devices. Video screen display 17 is useful for soft proofing(without a hard copy) and may be a high resolution video projectiondisplay for projecting images onto paper substrate(s) to be used involume reproduction with resolution sufficient to reveal moiré. i.e.,halftone frequency beating patterns. Note that proofing devices aretypically used to represent the performance of a client, such as aproduction rendering device described below at production node 104. TheCMI associated with each proofing device is referred to as a standardobserver meter (SOM) 13 and provides color measurement data from imagesfrom the proofing device. SOMs 13 have the capability of measuring coloras humans see it using the internationally accepted Standard Observermentioned earlier, and will be described in more detail later.

One of the pre-publishing functions supported by prototype node 102 isdesigning page layouts. Accordingly, a user or designer at nodes 102 caninput digital color graphical/image data from a storage 19, which may bea hard drive, or from other sources. The color image data may consist ofpage layouts having images at low and high resolutions, such as RGB. Auser at the node can define color preferences for rendering of the colorimage data, and later modify such preferences. The rendering of theinputted color image data at rendering devices to create soft or hardproofs is discussed later.

Production nodes 104 of network 11 control a production rendering devicevia the device's control system. Production rendering devices includevolume production machinery, such as press 15, which includes gravurepresses, offset presses, electrophotographic printing machines, ink jetprinters, flexographic presses, and the like. In addition, productionnodes 104 may also have one or more rendering devices and SOMs 13 of aprototype node 102, such as proofing devices 20, which allows proofingto occur at a production site. CMIs of node 104 are called imagicals.Like SOMs 13 of prototype nodes 102, imagicals 14 provide color data forimages rendered by press 15 in color coordinates of the StandardObserver. Proofing devices at prototype nodes 102 may also be outfittedwith imagicals 14 which may incorporate a SOM 13. The differencesbetween SOMs 13 and imagicals 14 will be described later. The maindistinctions between production rendering devices and proofing devicesare that the proofing devices are typically called upon to representsome other device, referred to herein as a client, such as printingpress 15 of a production node 104 and such presses may have interfacesto and mechanisms of control that are different from those of proofers.

At a production rendering device, the circuitry at node 104 differs fromnode 102 because it interfaces with inking control systems of theproduction rendering device to maintain its color quality during volumereproduction. This allows control of the actual marking process,including variables such as ink film thickness, toner density, and thelike, by supporting on-line colorimetry from a CMI within image areas ofprinted sheets. Analysis of the CMI data can be used to produce errorsignals in CIELAB color difference units or in other units suitable forinterface to commercially available inking control systems.

The nodes 102 and 104 each provide circuitry, which includes thecomputer and modem described above, for computation and communication.This circuitry operates responsive to programming of interface andapplication software stored at the nodes 102 and 104, and received usercommands at the nodes. The processes defined by such programming operatesystem 100. The circuitry perform several functions. First, it acceptsmeasurement data from CMIs and computes color transformation functionsto translate between human-perceptible colors of the measurement datainto rendering device colorant values. Second, it processes andtransmits color graphical/image data from one node or site in a network11 to another. Third, it can issue reading instructions to CMIs mountedon a rendering device to measure rendered color images, and issuerendering instructions to a rendering device at the node using a storedcolor transformation. Fourth, the circuitry performs communications insystem 100 in accordance with protocols for local or wide area networks,or telecommunications networks based on modem (either direct or mediatedby Internet connection—note that Internet connectivity is not limited tomodem,) satellite link. T1 or similar leased line technologies, ISDN.SMDS and related switched linkages, including Asynchronous TransferMode-enabled versions. TCP/IP, token ring, and the like. Fifth, thecircuitry implements calibration of rendering devices to a common, humanperceptible language of color, such as CIE, defined earlier, byproducing and storing color transformation information. Sixth, thecircuitry performs verification of the calibration of the renderingdevice to maintain accuracy of the stored color transformationinformation. These and other capabilities of the circuitry at a nodewill become apparent from the below discussion which describe furtherthe processes referred to above.

A feature of the present invention is a data structure operating withinsystem 100 called a Virtual Proof, hereafter called VP 12. The VP datastructure is a file structure for storing and transmitting filesrepresenting color transformation information between network 11 nodes.The contents of these files is outlined later. The VP is dynamic becauseit can be revised by nodes to assure the output color (colorants) of arendering device using data from CMIs. Preferably, the VP does notcontain color image data representing page layouts, and is associatedwith the page layouts. However, it can alternatively have files storingsome image data, although being separable from the often bulky highresolution image data representing the page layouts. The VP hascomponents or files shared by the nodes in network 11, and localcomponents or files present only at each node. Shared components arethose useful by more than one node in network 11, while local componentsare particular to information of each individual node's renderingdevice. Shared components are transmitted by the circuity of each nodeto other nodes of network 11 via pipe 11 a. Preferably. VP sharedcomponents are compact for transmission from node to node in network 11quickly. These shared VP components include files representing the usercolor preferences inputted at node 102 or 104, which is needed by eachnode in calibrating its rendering device. Each rendering device has itsown version of a VP stored at its associated node which represents theshared VP components and local components for that particular renderingdevice. In FIG. 3A, VP₁ VP₂, VP₃ and VP₄ represent the versions of thevirtual proof for each rendering device. The arrows from SOM 13 orimagical 14 represents the measurement data received by the nodeincorporated into color calibration data, which is stored in a localcomponent of the VP.

The VP provides system 100 with many useful features, which includeremote proofing for both intermediate and final approval of colorproducts, conferencing at multiple nodes in the network between userswhich may have different rendering devices, and distributing colorpreference data with or without page layout image data. The conferencingmentioned above allows users to negotiate over the colors appearing inpage layout and to confer about color corrections. For example,conferencing may use video displays 17 (soft proofs) of the page layoutsusing remote annotation software, such as imagexpo. Another importantfeature of the VP is that it is modifiable such that as changes occur ata rendering device, such as in inks or substrates, system 100 canautomatically adjust the rendering device's calibration. In addition,adjustments to calibration may be performed on demand by a user. Thisallows a user to update color preferences, such as color assignments ofpage layouts being rendered by rendering devices in the network withoutretransmitting the entirety of the image data.

A feature of system 100 is that it compensates for differences in thegamuts of different devices. As described earlier, a gamut is the subsetof humanly perceivable colors that may be captured or rendered by adevice. The preceding definition implies that the ideal or limitinggamut is the set of all colors that a normal human can see. It isimportant to distinguish between receptive and rendering gamuts. Theformer refers to the gamut of a sensor or camera of a CMI, or human. Thelatter refers to the colors that an output rendering device is capableof producing in a medium by application of its colorants. Although itmay be possible to design a rendering device that may produce all thecolors we can see under suitable circumstances, rendering gamuts aregenerally smaller than the human perceptual gamut due to the propertiesand limitations of practical reproduction media. For example, the gamutof a color print viewed in reflected light is generally smaller thanthat of a video display device viewed in controlled illumination whichis generally smaller than the gamut available with positive photographictransparencies. All the foregoing rendering gamuts are generally smallerthan receptive gamuts.

The CMI's of FIGS. 3B and 3C are colorimeters such as discrete (unitary)colorimeters (SOM 13) or imaging colorimeters (imagical 14) which may beused in conjunction with single-channel light-detectors. Thesecolorimeters may be stand-alone units or built-in to a rendering device.As stated earlier, the CMIs are controlled by their associated nodes insystem 100 for calibration and verification of rendering devices. SOMs13 and imagicals 14 are specialized to measure color in different ways.SOMs are suited for measuring a spatially homogeneous patch of color,preferably in a multiplicity of spectral bands. Preferably, at least 15spectral bands spanning the visible spectrum are sampled, making a SOMmore-than-3-channel input device. The transformation from relativespectral energy or reflectance to image colors employs the convolution,a similar technique is described in CIE Publication 15.2, page 23, citedearlier. An example of a SOM is a unitary colorimeter or aspectrophotometer of a U.S. Pat. No. 5,319,437 issued to Van Aken et al.

However, the distinction between SOMs and imagicals is not intrinsic. ASOM with a sufficiently restricted aperture and area of view and whichcould perform the spectral integrations sufficiently rapidly and whichcould scan rasters of image data may qualify as an imagical. Imagicalsare suited for multi-scale (i.e., variable resolution) imagingcolorimetry, consisting of an array of photosensors, such as CCDs,capable of sensing color, as would the Standard Observer.

SOM 13 is calibrated against a reference standard illumination sourcewhose calibration is traceable to the U.S. National Institute ofStandards and Technology or similar organization. The calibration of SOM13 is generally set by the factory producing the device. SOM 13 shouldbe periodically recalibrated to assure its reliability. Calibration ofimagicals will be described later.

Further, SOM 13 may be used in conjunction with an imagical. SOM 13 canprovide a check on the calibration of imagical 14 by sampling some ofthe same colors measured by the imagical and providing reference data tocompare against the imagical measurements. Under suitable circumstancesthe SOM enables a spectral interpretation of what is seen by theimagical so that a spectral illumination function characteristic ofviewing can be substituted for that used in measurement as described inconnection with FIG. 3D.

Referring to FIGS. 3B and 3C, preferred configurations for, sensors forCMIs in system 100 are shown. Because colorimetric accuracy of CMIs andtheir ease-of-use are desireable, the preferred configuration of CMIdevice colorimetric sensors is to have them be as unobtrusive aspossible to the user. Thus, in the preferred embodiment, CMIs arebuilt-in to the rendering device.

In the case of a conventional video display or monitor, FIG. 3B showsthe preferred embodiment for sensor of a CMI. A cowel 26 attaches to aupper chassis 27(a) to frame a faceplate or screen 24 of video display22 and to shield the display from most ambient illumination. A fiberoptic pickup (not shown) coupled to a projection type lens or lenssystem 28 plugs into cowel 26 in order to measure color of screen 24without actually touching the screen or requiring placement by the user.Path 30 shows that the line of sight of the lens and fiber optic pickupreflects off faceplate 24 and views the blackened inner surface of alower cowel 32 attached to lower chassis 27(b) such that it does not seespecularly reflected light reflected from faceplate 24. Nonetheless,operation of display 22 is preferably in an environment with subduedillumination.

Preferably the CMI for a display screen is as a unitary colorimeter SOM13. The unitary colorimeter takes color measurements via lens system 28when needed in response to instructions from circuitry at a node.Unitary colorimeter SOM 13 can measure relatively small areas of screen24 using a sensor connected to the fiber optic pickup. This sensor canbe a spectral sensor, a 3 or 4 filter colorimeter or a single channelsensor. A spectral sensor must be able to resolve 2 nanometer wavebandsacross at least the long wave (red) end of the spectrum in order to makean adequate measurement of the red phosphor of conventional CRTs. Thiscould be accomplished with a grating monochromator and linear photodiodearray, or linearly variable interference filter and diode array.Scanning the red end of the spectrum may be performed with a narrowly,electronically-tuned, spectrally variable optical filter in order tolocate red phosphor peaks exactly. If a 3 or 4 channel filtercolorimeter is used, compatibility with system 100 requires that thespectral sensitivity of the arrangement be a linear combination of thehuman color matching functions with acceptable precision. The videodisplay 22 phosphors change on a slow time scale. Therefore, providedthat the primary chromaticities of the display are measured on a regularschedule, the SOM of FIG. 3B may be replaced by a single-channel meterfor routine measurements of gamma (the voltage in—photons outcharacteristic of the device) very accurately for the three channels.

Alternatively, an imagical may be used instead of a unitary colorimeterSOM 13 in FIG. 3B. In this case the sensor of the imagical may becentered in a door (not shown) which is attached to cover the aperture29 of cowel 26 and 32 (the cowel may surround the entire perimeter ofthe chassis) so that the sensor views faceplate 24 center along a lineof sight. Imagical may acquire data needed to flatten the screen, i.e.,make it regionally homogeneous in light output or to compensate forinteractions among adjacent pixels in complex imagery. However, both ofthese factors are second-order effects.

It was noted earlier that the preferred embodiment utilizes a videodisplay 17 that projects the image, preferably onto printing paper. Inthis configuration, the CMI which monitors output is situated near thesource of projected light, as close as possible to being on a line ofsight. Preferably, a projection video display is capable of resolutionsexceeding 1200 lines per inch. In this way, it is possible to simulaterosettes, moires and details of image structure on the hard copy and toshow the actual structure of images captured from prints with a CMIequipped with macro optics. To provide high resolution projection,butting together a composite image using several limited-area-displaysmay be performed. Regardless of type of display 17, the display may useadditional primaries in order to extend gamut or better simulatesubtractive colorants.

FIG. 3C shows an example of a sensor of a CMI for measuring a substrate34 (hard proof), such as produced by a proofer device. In the case of adigital proofing device, such as a dye sublimation or ink jet printer,it is desirable to position a dual fiber optic pickup/illuminator 36arrayed for 45 and 90 degrees measurement geometry near the print headin order to sample recently printed color pixels of substrate 34.Pickup/illuminator 36 have projection-type lenses coupled to both asensor fiber and a illumination fiber within a light shielding sleeve toimplement non-contact measurements and to protect against stray light.Pickup 36 relays light to an analysis module of a CMI in which,preferably, a spectral colorimetric measurement is made. If placementnear the print head is not practical, then placement over the exit pathof printed sheets is preferred. This sort of placement is preferred forproofing devices which are page printers, such as electrophotographic orconventional, analog proofers. As was the case with monitors, the use ofa single-channel device to monitor linearization is compatible, providedthat the nature of the device variation over time is compatible withintermittent, full, colorimetric calibration. For example, it may besufficient to calibrate a dye sublimation proofer only once for eachchange of ribbon. Although it is preferred to build the CMI into theproofing device in order to minimize user involvement with the process,system 100 can alternatively require the user to place the printed copyon a X-Y stage equipped with the dual fiber optic pickup/illuminator 36.

At production node 104 of system 100, it is preferred that imagesrendered from a press are captured as soon as possible after they areprinted, i.e., after all the colorants are applied. In this way the datanecessary for control are available promptly. Although system 100 mayestimate the color-error of printed sheets relative to an aim value, thepreferred embodiment applies imaging colorimetry of a CMI to theanalysis of the image area. Preferably, there is a spectral component tothe CMI measurement such that interactions of the colorants with thestock or printing substrate can be analyzed and it is easier to computethe colors as they will appear under standardized viewing conditions.

Imagical 14 of a production node 104 may employ SpectraCube technology,reference cited earlier, or cameras using any filtering technology thatcan simulate Standard Observer response adequately. The preferredembodiment, imagical 14 has one or more cameras, such as solid statearea arrays, in which the light can be filtered through at least threewavebands, either simultaneously or sequentially, in conjunction with aunitary type colorimeter which views a determinable region of the imageviewed by the camera. This provides for the possibility of a spectralinterpretation of each pixel as needed. A spectral interpretation isdesirable in order to be able to control color to a criterion of how itwill appear under the final viewing illuminant. For example, an on-presscamera could be several cameras/sensors, each equipped with a relativelynarrow-band filter. The cameras, used in conjunction with a unitarycolorimeter, can sample composite spectral curves, as shown in FIG. 3D,in such a way that inference of the total spectral reflectance functionis possible. The minimum number of camera channels depends on the numberof colorants and the complexity of their spectral absorption curves. Thecameras may include an infrared-sensitive camera to differentiate theblack contribution to a spectral curve from contributions of non-neutralcolorants.

Preferably, imagical 14 is capable of multifocus or variable focusimaging so that larger areas of the image can be viewed at lowerresolution or vice versa. Views of small areas at high resolution enablesimulation of fine image structure by proofing devices capable ofsufficiently high resolution display. It is also preferred that imagical14 is equipped with anti-aliasing filters since much of the input to theimagical can be expected to be screened or pixellated. The article byGrievenkamp (cited earlier) describes an example of anti-aliasing. Alsopreferably, the viewing illuminant is controlled to simulate the viewingilluminant during the measurement. This may be achieved using aspectrally-adaptive, electronically tunable filter to match theillumination spectrum of any desired light source, which is described inreferences by Shnitser and Hoyt (cited earlier).

Ease of use and accuracy requirements indicate that CMIs are calibratedor self calibrating in the preferred embodiment. The preferredembodiment of the unitary device SOM 13 approximates a dual-beam device,such as spectrophotometer of Van Aken et al. cited earlier. The spectrumof light reflected from the unknown sample is compared eithersimultaneously or successively with the light of the same sourcereflected from a known reflector. In this way it is possible to separatethe spectral contributions of colorants, substrates and illuminationsources and to estimate their true functional forms over multipleimpressions. This increases the CMI's accuracy. Even with suchmeasuring, however, regular recycling of instruments for factoryre-calibration or field recalibration using standardized reflectanceforms should be performed. Also, if video display 17 is a self-emissivemonitor, the above dual-beam functionality although not useful incomputing a difference spectrum, provides a means of evaluating possibledrift or need for re-calibration in the instrument.

System 100 operates in accordance with software operating at the nodes,which is preferably based on object oriented coding, a well knownprogramming technique. However, other programming techniques may also beused. The discussion below considers the different input and outputdevices, e.g., CMIs and rendering devices, as objects. Each objectrefers to software applications or routines in system 100 which providesinput to or accepts output from other applications (or devices).

Referring to FIG. 4A, a three-dimensional matrix model of the abstractclass device/profile is shown. For example, a device may be instantiatedas a linear input device or non-linear output device and deriveproperties through inheritance. An object created from a certain classis called an instance, and instances inherit the attributes of theirclass. Inheritance is a functionality of object-oriented coding andprovides for grouping objects having common characteristics into a classor subclass. The matrix is extended in a third dimension to include3-colorant-channel devices and more-than-3-colorant-channel devices.Display devices may have 4 or more colorant channels (for example, Red,Green, Cyan, Blue, indicated by RGCB Monitor 38) in order to enhancegamut or better simulate subtractive color reproduction processes. Inother circumstances, a display device may appear to fall into the sameclass as a CMYK printer 39, which it represents in asoft-proofing/client relationship. The appropriate models and transformsare those associated by inheritance with the subclass into which the newdevice falls within the general matrix.

A CMYK printer 39 exemplifies the (sub)classoutput/non-linear/more-than-three-channel. By inheritance,client-proofer relationships are shared by members of subclass output,since the ability to enter into client-proofer relationships can bepassed to all members of the subclass by inheritance. Note that thesubclass of input devices is distinguished by conservation of gamutamong its linear members. Likewise, specific instances of subclasslinear inherit an association with linear matrix models of colortransformation, whereas non-linear subclass members associate with colortransformation by polynomial evaluation, interpolation or other form ofnon-linear color mixture function. Procedures performing the colortransformations are incorporated in the data structures defining theobjects, and are discussed later in more detail. More-than-three-channeldevices require procedures for rendering with the extra colorants, whichis also discussed later in more detail.

Note that the class hierarchy depicted herein supports instantiation ofdevices of types “scnr” “mntr” and “prtr” as promulgated by the priorart ICC Profile Specification (cited earlier). However, the classhierarchy disclosed herein is considerably more general, flexible andextensible. The properties of flexibility and extensibility areillustrated by the following practical example: a video display (“mntr”in prior art ICC Profile Spec) in system 100 may occupy any of a numberof cells within the class structure depending on its physical properties(for example, the number of colorant channels it has) and purpose(stand-alone design workstation or soft proofer representing afour-colorant device.)

The term “linear” herein, when applied to a device, means that linearmodels of color mixture can successfully be applied to the device (seepreviously cited art by Gordon and Holub and by Holub.) It does notimply that a video display is intrinsically linear. For example, amonginput devices, linear defines that linear color mixture models can beused to convert from CIE TriStimulus Values to linearized (or gammacompensated) device signals and vice versa. Further note, that oneapplication can be an output device with respect to another. Forinstance, application software may convert RGB TriStimulus Values intoCMYK colorants and occupy the same cell as a CMYK printer 39.

The calibration of devices in system 100 is indicated by the classes ofthe devices 40, 41, 42 and 39 in FIG. 4A. Calibration herein includesthe process of obtaining color transformation data in uniform colorspace. Once devices at nodes in a configured network 11 of system 100are calibrated, the colorants produced by nodal rendering devices canthen be controlled, however such calibration remains subject torecalibration or verification processes, as described later. There arefour classes of devices 40, 41, 42 and 39 which require calibration:

1) imaging colorimeters or imagicals 14 (class input/linear/3-channel);

2) video displays 17 (generally in class output/linear/3-channel);

3) unitary, spectral colorimeters or SOM 13 (classinput/linear/more-than-3-channel); and

4) printers or presses (generally in classoutput/non-linear/more-than-3-channel).

Optionally, non-linear input devices may be used in system 100, asdescribed in the article by Sodergard cited earlier, but are lesspreferred. An example of a procedure for calibrating non-linear inputdevices to a colorimetric standard is described in Appendix B of theAmerican National Standard “Graphic technology—Color reflection targetfor input scanner calibration” (ANSI IT8.712-1993).

In the first class of devices, the calibration of imagicals 14 involvespreparing compensation functions for the separable non-linearities ofthe device's transfer function, which are usually addressed in eachcolor channel individually. These compensation functions may be realizedin one-dimensional look-up-tables (LUT), one for each color channel. Thecompensation functions may be defined in a calibration step in whichmeasurement signals from imagical 14 are generated in response toobservation of step wedges of known densities. Next, specifying theconstant coefficients of a linear color mixture model expressed as a 3×3matrix transformation, hereinafter referred to as matrix M, whichconverts linearized device codes into CIE TriStimulus Values, such asXYZ, or related quantities. The formation of matrix M is described inGordon and Holub (cited earlier). Last, the gamut of the input is scaledto fit within the color coordinate scheme in which images arerepresented. Because inputs to the system are printed copy (proofs andpress sheets,) gamut scaling is often unnecessary except when the imagerepresentation is a space such as calibrated, monitor RGB which may notencompass all the colors of the print. Occasions on which this wouldmost likely be a problem are those involving extra colorants used for“Hi Fi” effects, although limited ranges of conventional printing cyansand yellows are out-of-gamut for some monitors. Preferably, imagicals 14are self-calibrating to assure the accuracy of their color measurements.The compensation function LUTs, matrix M, and possible gamut scalingdata are considered the calibration transforms for each imagical 14.

The color transformation in imagicals 14 into uniform color space basedon the above calibration is generally shown in FIG. 4B. Imagicals 14output measurement signals R. G and B, also referred to as device codes.The measurement signals are passed through compensation function LUTs 48(or interpolation tables) to provide linearized signals R_(lin), G_(lin)and B_(lin), in order to compensate for any non-linearities in therelationship between light intensity sensed by imagical 14 and thedevice codes. Matrix M then operates on the linearized signals R_(lin),G_(lin) and B_(lin) to provide X, Y, and Z coordinates. Matrix M isshown consisting of the 3×3 coefficients (a₀₀-a₂₂) defining the linearcombinations of R_(lin), G_(lin) and B_(lin) needed to match X, Y and Z,the TriStimulus Values (TSVs) of the CIE Standard Observer. Although notshown in FIG. 4B, gamut scaling is performed after TSVs are converted toUniform Color Space coordinates such as those of CIELAB. Scaling of aninput gamut onto an output gamut in this case is entirely equivalent toprocesses detailed for rendering devices later in this description.

Calibration of video displays 17 in the second class of devices followsthe same steps for imagicals 14 described above, however since videodisplays 17 are output rendering devices, matrix M and compensationfunction LUTs are inverted. Calibration of video displays for softproofing is well known, and discussed by Holub, et al. (J. Imag.Technol., already cited.)

Referring to FIG. 4C, device independent color coordinates XYZ are inputsignals to display 17. Rendering employs the inverse of the operationsused for imagical 14. The inverse of the calibration matrix M is calledA⁻¹ (to emphasize that we are considering numerically different matricesfor the two devices) and is used to convert the XYZ input signals tolinear device signals R′_(lin), G′_(lin) and B′_(lin) The linear devicesignals R′_(lin) G′_(lin) and B′_(lin) are postconditioned using theinverse of the compensation function LUTs which define the non-linearrelationship between applied signal and luminous output of display 17, afunction which is defined and adjusted in a separate, empirical step ofcalibration. The output from the LUTs are gamma corrected signalsR^(1/y), G^(1/y), and B^(1/y) representing the input to display 17. Notethat there is no necessary relationship between the matrices A⁻¹ and Min FIGS. 4B and 4C. Further, since the LUTs of FIGS. 4B and 4C may beused with various types of transformations in system 100, they arepreferably represented by a separate data structure, within the softwarearchitecture, which may be combined like building blocks with otherstructures, such as 3×3 matrix, or multidimensional interpolation table,to form more complex data structures.

As stated earlier video displays 17 generally belong to the subclassoutput/linear/3-channel in FIG. 4A. However, there are two importantexceptions to this: a) when display 17 is used to represent amore-than-3-channel printer, the transforms used to drive the displayare non-linear—in other words, a proofing device manifests attributes ofits client; and b) when display 17 is used as an accomplice in thecreation of computer-generated art, the video display can be considereda linear input device, since new, digital, RGB data are created in themedium and color space of the display. Likewise, calibration for a RGCBMonitor (device 38 of class linear/output/>3-channel) is asimplification of procedures for calibrating the class ofnon-linear/output/>3-channel devices, which is described below.

In the third class of devices, the calibration of unitary, spectralcolorimeters or SOM 13 is set by the factory, as discussed earlier, andthus does not require the preparation of calibration data to providedevice independent color coordinates.

Referring to FIG. 5, the process of calibrating the fourth class ofdevices is shown. It should first be noted that for proofing devices inorder to represent one device on another (to proof) it is necessary tohave an accurate model of the color reproduction of both devices. It ispreferred that the proofing device has a larger gamut than the clientand that accurate knowledge of gamut boundaries be derivable from themodels of colorant mixture. Because proofing is an issue with outputdevices, it is necessary to develop rendering transformations thatinvert the models of colorant mixture and that mix colorants on theproofer in such a way as to match the colors produced by the client asclosely as possible. In other words, this should involve respecting thegamut limitations of the client device when rendering on the proofer. Insummary, the following four kinds of color transformations are developedin calibration of the fourth class of devices:

1) forward models that enable calculation of the color, in deviceindependent coordinates, of a mixture of colorants;

2) forward model inverses that enable calculation of the amounts ofcolorants needed to render a desired device independent colorcoordinate;

3) descriptions of gamuts in terms of boundaries specified in deviceindependent color coordinates; and

4) mappings of colors realizable on one device onto those realizable onanother in a common, device independent coordinate system (gamutconfiguration data).

The above four color transformations are discussed in more detail below.The following is in reference to a hard copy proofing device or proofer,but is applicable to other devices in the fourth class, including highand low volume presses.

Step 1 of FIG. 5 is the process of preparing linearization functions,which is shown in more detail in FIG. 6A. This process establishes alinear relationship between the digital codes sent to the proofer andthe output of the proofer, measured in units such as visual printdensity. Linearization improves the utilization of the available digitalresolution and is usually implemented by means of one-dimensional LookUp Tables (LUTs) that map linear digital codes from the nodal computeronto signals that drive the marking engine of the proofer to produce anoutput that is approximately linear. For example, step wedges printed inC, M, Y and K respectively should produce gradations in measurablevisual density that increase linearly as a function of the digital codescommanded by the host.

Usually, linearization involves printing a step wedge on the markingengine without the benefit of the LUT—if data are passed through theLUT, it applies an identity mapping. The color samples of the wedge areanalyzed by the CMI associated with the proofer and the measurements aresupplied to the nodal processor so that it can compute the transferfunction from command codes to print densities. The measured transferfunction is compared to the desired one and a function that compensatesfor the errors in the measured function is prepared—this is what isloaded into the LUT for use in normal image transmission. The LUTs arewritten to the local part of the VP as linearization functions.

Linearization is not a strict prerequisite for the remaining proceduresbecause a multidimensional color transformation could be designed toaccomplish what the one-dimensional LUTs are supposed to do. Thus,linearization of step 1 although preferred in system 100, may optionallybe incorporated into other color transformations in FIG. 5. However, itis generally advantageous to exclude as many sources of non-linearity aspossible from the transformation function which is specified by theprocedures outlined here.

Step 2 of FIG. 5 involves verifying or renewing the calibration of theCMI and rendering calibration forms, and is described in the flow chartof FIG. 6B. After initializing calibration procedures, if the CMIassociated with the rendering device is an imagical 14, it is calibratedto provide calibration transforms, as described above. Preferablycalibration of the CMI is performed automatically in response toinstructions from circuitry at the node.

After the CMI associated with the rendering device is calibrated, acalibration form is rendered on the proofer. For example, this form mayhave the following attributes: 1) a sampling of all combinations of fourlevels of all of the colorants, 2) inclusion of approximately neutralstep wedges. 3) several samples of flesh tones, 4) a number of redundantpatches—these are common inkings placed at different locations on theproof to provide information about spatial non-uniformities of theproofing process. It also is useful to include at least short stepwedges in each of the colorants and their overlaps, for example, cyan,magenta and yellow as well as blue (cyan+ magenta,) green (cyan+ yellow)and red (magenta+ yellow.)

The calibration form described consists of about 300 samples in the caseof 4 colorants and can fit on an 8.5×11 inch (21.5×28 cm) sheet withpatch sizes of 1 cm on a side. However, generally, the number of patchesshould be three times the number of polynomial terms fitted to the data(discussed later in step 4, FIG. 8.) Patch sizes are scaleable forcompatibility with various CMIs. In addition to the tint samples, thetarget has markings similar to pin registration marks and demarcatedhandling zones to facilitate transfer of the target from the proofer tothe CMI if the CMI is not incorporated into the proofer. Theregistration marks indicate clearly where and how to insert the copy ina stand-alone CMI so that the instrument will find the patches wherethey should be and the handling zones will emphasize to the user thatthe image area should not be touched. Hard copy proofers may write anidentifying number and/or bar code for the device and for the specificproof (including date and time) on the proof.

Alternatively, a device in the fourth class, such as a running press,may be calibrated by analysis of live imagery (rather than a calibrationform) digitized by an imagical provided 1) that the images analyzedsample the gamut of the device adequately and 2) that the effects ofpage adjacency within a signature on color reproduction can be accountedfor (for example by reference to stored historical data at the node.) Asalways, the relevant data for calibration are the known colorantspecifications embodied on the printing plate and the colors resultingon the printed sheets.

After rendering the calibration form, the form is measured by a CMI andcalibration data is provided (Step 3 of FIG. 5). The processes of step 3are detailed in the flow chart of FIG. 7. As stated earlier, thepreferred CMI is a linear colorimeter, such as a imaging or unitarycolorimeter, which is capable of providing spectral analysis data whichsupports illuminant substitution. The CMI should produce severalreadings of color to the node; if one is an outlyer compared to theothers because of a blemish on the copy or some malfunction, then thesoftware at the node excludes it from the averaging for that patch. Ifno two of the measurements agree, then the patch should be flagged so asto identify it as a problem in subsequent processing. More generally,the number of measurements of each patch is odd to permit voting by thesoftware in the interest of selecting trustworthy measurements.

If imaging of the calibration form is performed by an imagingcolorimeter or imagical 14, then the imaging colorimeter analyses theimages on the form, and uses its calibration transforms from step 2 tocalculate image colors and standard error of measurement in CIE UniformCoordinates, such as L*, a*, b*. Also the imaging colorimeter providesmany sample values of each patch color; regional checks fornonuniformity of color in the sampled area should be performed. However,if imaging of the calibration form is performed by an unitarycolorimeter or SOM 13, then the patch readings from the form areconverted to color measurements in CIE Uniform Coordinates.

Each patch measurement may include information about sensitivity,integration time, wavelengths sampled, illuminant substitutions, and thelike. The series of measurements from each calibration form areaccompanied by at least one record of the reference spectrum, although,obviously, reference spectral data will be collected and used on everyreading, at least for reflection measurements.

Regardless of the type of CMI, a list of colorant values (theIndependent Variables. IVs, to the fitting procedure) to correspondingcolor values (Dependent Variable) with standard deviation are assembled.On this list of measurements outlyers are flagged. An estimate of sheetvariation from redundant sampling is produced.

In addition to multiple measurements within a given sheet, two otherMeans for enhancing the reliability of the data are provided. First, thesoftware supports measurements of multiple sheets and second, anhistorical record of measurements from a particular proofer or press ispreferably maintained at a node. Historical data can be stored morecompactly and compared more readily to current data if the measurementsare converted from spectral form to colorimetric form. Spectral data ofthe measurement is stored in a database at the node in terms of colorand summary statistics.

Preferably, the database maintains a FIFO history of color and summarystatistics from the most recently measure forms. Because step 5 involvesleast squares error minimization, flagging of outlyers is preferred toreduce the influence of one bad reading. A decision on whether a currentreading is legitimate is made by comparing CIE ΔE* values rather thanthe two spectra. The assembled list with flagged outlyers and standarddeviation of each patch measurement is written to a calibration (cal.)data file in the local part of the VP, for later use in building theforward model.

After step 3 is complete, processing continues to step 4 of FIG. 5,building a forward model based on the calibration of step 3. Step 4 isflow charted in FIG. 8. This model will represent color as a function ofcolorants of the proofer or press. It should first be noted that genericpolynomials provide a satisfactory form for models of color generationby colorant mixture on printing devices. However, this does not excludeany other mathematical or physical modeling procedure capable ofproducing transformation functions of sufficient colorimetric accuracy.Polynomials of relatively low order in each of the colorant variablesmay be, fitted very nearly to within the device variation. In otherwords, the uncertainty of the model prediction is not much greater thanthe uncertainty of color rendered in response to a given set of digitalcodes. Low order implies that the colorant variables are not representedas powers greater than 2 or 3 and that the sum of the powers of theindependent variables in a single term of the polynomial is limited to4. Therefore, if the inks C, for cyan, M, for magenta, Y, for yellow,and K, for black are the independent variables, a valid term could beC²MK, but not C²M²K. Analytical derivatives are easily computed, whichmakes them advantageous for model inversion.

A polynomial forward model is fitted to a data set consisting ofindependent variables of colorant and dependent variables of deviceindependent color coordinates (stored in the calibration data file ofthe VP) by the method of least squares. A polynomial is a linearcombination of each of its terms which are called, mathematically, basisfunctions at step 82. Without loss of generality, discussion can besimplified by considering functions of two variables, C and M, in whicheach variable may be raised to a power of up to 2 and the sum of powersmay not exceed 4:

R=a ₀₀ +a ₁₀ C+a ₂₀ C ² +a ₀₁ M+a ₁₁ CM+a ₂₁ C ² M+a ₀₂ M ² +a ₁₂ CM ²+a ₂₂ C ² M ²

G=b ₀₀ +b ₁₀ C+b ₂₀ C ² +b ₀₁ M+b ₁₁ CM+b ₂₁ C ² M+b ₀₂ M ² +b ₁₂ CM ²+b ₂₂ C ² M ²

B=c ₀₀ +c ₁₀ C+c ₂₀ C ² +c ₀₁ M+c ₁₁ CM+c ₂₁ C ² M+c ₀₂ M ² +c ₁₂ CM ²+c ₂₂ C ² M ²

In the foregoing, color is the vector valued function ¦R G B¦ of thevariables C and M and the a's, b's and c's are constant coefficientswhich give the proportions of their corresponding terms to be mixed informing the linear combination. The purpose of the fitting procedure isto find the set of coefficients which results in the least squared errorwhen comparing the color measured at a given patch with the colorcalculated by substituting the patch's colorant values into the forwardmodel. The polynomials are untruncated within the constraints on thepowers. The DV may also be L*, a*, b* coordinates of CIELAB uniformcolor space.

In FIG. 8, the process of building a forward model begins by building adesign matrix for the problem (step 82). The design matrix has Mcolumns, one for each basis function, and N rows, one for each patchmeasurement. The number of rows should exceed the number of columns;practically, the ratio of N to M should be greater than 3 for goodresults. After reading the cal, data file, each cell in the matrix isfilled with the value of the basis function for the column atindependent variables (inks) given by the row, divided by the standarddeviation of the patch measurements, if available, else by 1 (step 83).Note that the patch measurements themselves do not enter the designmatrix. Then use the design matrix and vectors of patch measurements foreach of the three, color, dependent variable dimensions to write matrixequations that can be solved for the desired coefficient vectorspreferably by Singular Value Decomposition. SVD (step 84).

The numerical methods outlined in the preceding and the followingparagraphs are similar to those described by Press, et al. (NumericalRecipes. Cambridge University Press. Cambridge, UK. 1986.) Sections14.3. “General Linear Least Squares.” with SVD fitting, and 5.3.“Polynomials and Rational Functions”.

The model and its derivatives can be evaluated efficiently by a methodof recursive factorization. The method depends on use of polynomialterms as basis functions. However, it permits evaluation of the functionwith no more than 1 multiplication and 1 addition per polynomial term.Because the independent variables never need to be raised to a power,the demands for precision on the computational machinery are not asgreat. The principle can be seen most readily in one dimension; thefunction y=a₀+a₁x+a₂x² can be factored to yield a₀+x(a₁+a₂x) whichevaluates with 2 multiplies and 2 adds. Generalizing this to thetwo-dimensional function described above:

a ₀₀ +a ₁₀ C+a ₂₀ C ² +M(a ₀₁ +a ₁₁ C+a ₂₁ C ²)+M ²(a ₀₂ +a ₁₂ C+a ₂₂ C²),

or

a ₀₀ +C(a ₁₀ +a ₂₀ C)+M[(a ₀₁ +C(a ₁₁ +a ₂₁ C))+(a ₀₂ +C(a ₁₂ +a ₂₂C))M].

How to generalize to three or four dimensions is apparent.

At step 81, the patches that were flagged during measurement as outlyersare excluded from the fitting. The fitting program estimates thevariation in the device based on deviations of color in redundantpatches and/or measurements of multiple copies. The average error of fitis calculated as the average ΔE* (CIE Color Difference Unit) over theset of measurements of patch colors compared to the colors predicted bythe fitted polynomial (step 83). This average should not exceed theestimated device variation by more than 1 ΔE* unit. When it does, orwhen the software detects individual patch discrepancies exceeding 4 or5 ΔE units, the software flags apparently outlying points (step 85). Itthen computes trial fittings with outlyers omitted to see if averageerror can be improved (step 86). It also is informed with a strategy forapplying various sets of basis functions in the interest of achieving anacceptable fit. However, the fitting procedure will reject a dataset atstep 86 rather than get too involved with it.

Principal Component Analysis, or an equivalent method, may be employedto reduce the number of polynomial terms (complexity of the model)consistent with a given average error criterion. This technique issimilar to those described in Johnson and Wichern. Applied MultivariateStatistical Analysis. 3rd ed., Englewood Cliffs. N.J.: Prentice Hall.1992, ch. 8.

Fitting concludes with writing a final polynomial model descriptor (adata structure and a file) consisting of a header and lists ofcoefficients. The header includes information such as the identity(ies)of proofs measured, relevant dates and times, the ingredient data files,the form of the polynomial (maximum powers of the independent variablesand maximum order of a term) goodness of fit statistics. The polynomialdescriptor will be needed later by the polynomial evaluator of thesoftware and is written into the shared portion of the Virtual Proof.

Although this discussion is directed to a 4-colorant printer or press,the polynomial forward model is, potentially, part of a soft proofingtransform which enables a video display to represent a client printer.It can be used to compute a colorant_(A) to colorant_(B) transformationfor what is effectively a CMYK, subtractive-color video display, in thatCMYK colorant values are processed through a transformation to producedevice-specific RGB for a monitor. This can generalize to use ofmore-than-four-colorant-printers and more-than-three-colorant displays.

After the polynomial model descriptors are computed, a forward modeltable (FMT) and prototype gamut descriptor data are prepared (step 5 ofFIG. 5). The processes of step 5 are detailed in the flow chart shown inFIG. 9A. As described above, the polynomial based on the modeldescriptors represents the forward model. The forward model enables usto predict the colors resulting when certain colorant mixtures are askedfor on the printer or press. System 100 includes a polynomial evaluatorto evaluate this polynomial either in hardware circuitry or software atthe node. Referring to FIG. 9B, a topology of operators is shown(multipliers 86 and adders 87) to implement the polynomial evaluator forthe two-colorant case, cyan (c) and magenta (m). Each operator receivestwo inputs. Inputs (operands) designated by two numerals are theconstant coefficients and inputs designated by C 89 or M 90 stand forthe independent variables, amounts of colorant. For instance. 21 88denotes the coefficient which corresponds to the second power of C timesM. In order to evaluate a function of three variables, 3 of the unitsshown in FIG. 9B are needed. Four variables require 27 such units for anuntruncated polynomial. In a hardware realization of the evaluator, itis preferable to retain the generality of the polynomial form and zeropoly terms (or their coefficients) that are missing due to truncation.If 27 units are too many for a cost-effective device, then thecalculation may be staged and the successive stages pipelined through asubset of the 27 units, albeit at some cost in control logic and speed.Given the opportunities for parallelism in a hardware implementation,the preferred embodiment benefits by a chip to transform ink values tocolors at video rates. Such a chip may be a component of nodal circuitryencompassing a graphics accelerator to drive a video display device forsoft proofing at the node. It accelerates the generation of colorseparation transformations because evaluation of the colorant to colormodel is a major factor in that computation. It also accelerates theevaluation of color separation transformations in situations where thedata of the inverse, color-to-colorant conversion can be fitted bypolynomials with a sufficiently small average error of fit. Data forfitting can be the addresses and entries of an interpolation table ofsufficient size, as discussed below.

The FMT stores the results of the polynomial evaluator in a datastructure and colorant quantization/addressing scheme shown in the CMYKhypercube of FIG. 9C, which contains the entire gamut of colorsreproducible by a CMYK printer. The hypercube may be subdivided into aFMT having 17 points (16 intervals) per colorant dimension forsufficient accuracy. However, the software architecture supports more orless, within the constraint that the numbers of grid points perdimension satisfy the equation 2^(n)+1, where n is integer. Depending onthe requirements of the rendering applications and equipment, a softwareswitch controls whether the tables are written in pixel interleaved(each address of a single table holds the values of all dependentvariables) or frame interleaved format. In the latter case, threeM-dimensional tables are prepared, where M is the number of colorants.Each “cell” of a table has an M-dimensional address and contains a colorcoordinate computed by evaluation of the forward model at step 97 ofFIG. 9A. At step 97 nested looping over all colorant addresses isperformed and computed colors from the forward model are stored at eachaddress. Thus, each model evaluation yields three color coordinates eachof which is deposited in a cell corresponding to the M colorants whichproduce it in the appropriate table. Colors of inkings in the midst of ahypercuboid are estimated by interpolation.

Referring now to FIG. 9D, each of the M channels of input to the FMT maybe provided with preconditioning LUT for each of the IndependentVariables and each output channel may be processed through a1-dimensional postconditioning LUT. Interpolation is preferablyperformed by linear interpolation in the nodal circuitry or software.

The data structure in FIG. 9D accommodates the preconditioning of jaddress variables by j 1-dimensional transformations which may beimplemented as look-up tables with or without interpolation 98. Thepreconditioning transformation may pass one or more of the j inputsIndependent Variables (IVs) through to the multidimensional transformunaltered (identity transformation) or may apply a functional mappingsuch as a logarithmic conversion. The multidimensional transform 94 hasj input variables and i outputs. The preferred implementation of thetransformation is by interpolation in a sparse table of function valuesor by evaluation, in hardware, of a set of polynomials fitted to thetabular values (where fitting can be done with sufficient accuracy.) InFIG. 9D, a 3-dimensional IV 93 is applied to the multidimensionaltransform 94. Multidimensional transform 94 consists of many smallercuboids 95 whose corner points are the sparsely sampled values of the IVat which values of one (or more) of the dimensions of the dependentvariable, DV, 96 are stored. The IV provides addresses and the DVcontents. The subcuboid 95 is shown at the origin of the addressingscheme. Interpolation is used to estimate values of the DV occurring atvalue of the IV which are between corner points.

The data structure of FIG. 9D also accommodates the post-conditioning ofthe i output variables (DVs) of the transformation by i 1-dimensionaltransformations which may be implemented as look-up tables with orwithout interpolation 98. One of the transforming functions which may beincluded in the post-conditioning is the linearization functionoptionally produced by step 1 of FIG. 5.

The purposes of the 1-dimensional pre- and post-conditioning functionsinclude improving the extent to which the relationship between variablesinput to and output from the multidimensional transformation 94 isapproximated by whatever interpolation function is employed inevaluating the transformation. Therefore, the form of the functionsshould be known from preliminary studies of the device and the pre- andpost-conditioning transforms must be in place during calculations ofSteps 2 through 9 if they are to be used at all. For example, alinearization function which may be defined in step 1 should be used inrendering the calibration target in step 2.

A linear interpolation formula for two dimensions which generalizes tohigher dimensions with a simple, proportional scaling in the number ofoperations is described in Gallagher, “Finite Element Analysis”, citedearlier. For example, given a cell in a two-dimensional array ofsparsely sampled points as shown in FIG. 9E, the interpolated value of afunction f(x,y) at a point (x,y) interior to the cell may be calculatedas the weighted average of the endpoints in the direction of the longerof the components x or y plus the weighted average of the endpoints inthe direction of the second longest component. In other words, order thedistances of the point with respect to the axes of the cell and then sumthe fractional distances along those ordered dimensions. Written as anequation:

for y<x,f(x,y)=f(0,0)+x*(f(1,0)−f(0,0))+y*(f(1,1)−f(1,0)),

and

for x>=y,f(x,y)=f(0,0)+y*(f(0,1)−f(0,0))+x*(f(1,1)−f(0,1)).

FIG. 9A also flowcharts the preparation of the prototype gamutdescriptor (GD) data. As colorant addresses are converted into colors topopulate the FMT (step 97), the colors are also converted intocylindrical coordinates of CIE hue angle, chroma and lightness. Hueangle and lightness are quantized to become addresses into a 2-D gamutdescriptor, preferably dimensioned at least 128×128 for adequateresolution (step 99). The Chroma component of the color is notquantized, but is stored in linked lists of Chroma values associatedwith each hue angle, lightness coordinate. The finished gamut descriptordata is prepared from the prototype GD data later at step 7 of FIG. 5,and consists only of surface chroma values for each coordinate.Therefore, the prototype is not a shared file and is written to thelocal portion of the VP, while the FMT is written to the shareableportion.

Next, at step 6 of FIG. 5, the FMT is inverted into a prototypetransformation table, also called Proto SEP table. Rendering image at arendering device requires this inversion step. i.e. finding the colorantmixtures to realize a desired color which may be given in deviceindependent coordinates or in the color coordinate system of some otherdevice (such as the CMYK monitor mentioned above.) Due to the complexityof the problem, it is generally not feasible to perform forward modelinversion in real time as imagery is rendered. Practical approaches relyon interpolation in a sparse matrix of inverse function values. However,offering interactive control over key aspects of the separationtransformation (also called SEP) implies that at least some parts of thecalculation occur nearly in real time. Because inversion of the forwardmodel involves evaluating it, potentially numerous times, acceptableperformance requires a very rapid means of evaluating the forward model.One method involves design of specialized hardware for polynomialevaluation. When specialized hardware is unavailable, softwareperformance can be significantly enhanced via the same strategy as usedfor speeding rendering transformations: interpolate in a sparse table offorward model values, in particular in the FMT.

The proto SEP represents a color to colorant transformation with colorcoordinates as inputs and inkings as outputs. For illustration purposes,we will consider the case in which color coordinates are expressed inCIELAB units, L*, a* and b*. In order to build a sparse table of inversefunction values for the proto SEP, the following addressing scheme isdefined. If address variables, or inputs, are represented at 8-bitresolution, the span of the table in each dimension is 0-255 (2⁸)digital levels. Continuous perceptual dimensions are mapped onto thoselevels so as to satisfy two criteria: a) the entirety of the input andoutput gamuts of interest must be represented, and b) adjacent digitallevels are not perceptually distinguishable. Input and output gamutswill be detailed later; briefly, an input gamut is that of a device orapplication which supplies images (in this case for separation) and anoutput gamut is that of a device or application which receives theimage—in this case for rendering. With the above addressing scheme, aprototype separation table is computed. The following outlines thegeneral process for building the proto SEP table.

For each grid point, or address, of the table, the amounts of thecolorants needed to produce the input color are found. There are Ntables, one for each of N colorants, assuming frame interleavedformatting. First, a starting point of the correct inking is speculated.Calculate its color by the forward model and compare the calculatedcolor to the desired, address color. Save the color error, modify theinking, recalculate the color and see if the error is reduced. Theforward model is also employed to calculate the partial derivatives ofcolor with respect to ink. The resulting matrix, shown in the equationbelow, yields the direction of the movement in color occasioned by thechange in colorants. The partials of color error with respect to each ofthe colorants indicate whether movement is in the correct direction. Ifnegative, this indicates movement in the correct direction along thatink zero; then store each of the inkings in their respective tables.

da*¦∂a*/∂C,∂a*/∂M,∂a*/∂Y,∂a*/∂K¦dC

db*=¦∂b*/∂C,∂b*/∂M,∂b*/∂Y,∂b*/∂K¦*dC

dL*¦∂L*/∂C,∂L*/∂M,∂L*/∂Y,∂L*/∂K¦dYdK

The foregoing paragraph gives a simplified explanation of the algorithmembodied in procedures which use Newton's Method for finding roots(“Newton-Raphson”) or one of many optimization methods for searching out“best” solutions in linear (e.g., the Simplex method of linearprogramming) or non-linear multidimensional spaces. The algorithm canalso be implemented with neural networks, fuzzy logic or with the aid ofcontent addressable memory, as detailed in prior art by Holub and Rose,cited earlier. A neural network can be trained on the results ofrepeated evaluations of a mathematical model. Further, neural networksmay also be utilized to perform any of the color transformations of FIG.5. e.g., building the forward model or rendering transform table. Colortransformation with neural networks is described, for example, in U.S.Pat. No. 5,200,816, cited earlier. Newton's method is applied to findthe root of an error function. i.e., the solution is at the point atwhich the derivative of the error function goes to zero. Strictly.Newton's method is only applicable in situations where a solution isknown to exist. Therefore, although it is an efficient procedure, it issupplemented by other optimization methods, such as “SimulatedAnnealing.” (The model inversion technique described above is similar tothose in Press. et al. sections 9.6, 10.8 and 10.9, respectively,already cited.)

Optimization methods can produce a useful result when no exact solutionexists, as in the case when the desired color is unprintable by (or outof the gamut of) the device. The optimization is driven by an errorfunction which is minimized, typically by using downhill, or gradientsearch procedures. In the case of Newton's Method, one of the colorantvariables (in the four colorant case) must be fixed in order to find anexact solution; otherwise, there are infinitely many solutions. Usually,black is fixed. The algorithm for model inversion uses supporting searchprocedures in case the primary technique fails to converge to asolution.

The foregoing, simplified account overlooks the possibility that thegradient surface on the way to the solution has local minima in it whichthwart the search procedure. This risk is minimized by the presentinventors' technique of using the FMT to provide starting points whichare very close to the solution. The above process for building the protoSEP Table is applied in system 100 as shown in the flow chart of step 6in FIG. 10A. For each color entry in the FMT, find the closest coloraddress in the prototype separation table (step 111). Then use thecolorant address of the forward model table as the starting point in asearch for a more exact ink solution at that color address of the protoSEP (step 112). The search and the addressing of the interpolation tablecan be in the cylindrical coordinates of hue, chroma and lightness or inthe Cartesian coordinates L*, a* and b*. In the preferred embodiment,the forward model table has at most 17×17×17×17˜=84K grid points and theprototype separation table has at most 33×33×33˜=35K grid points many ofwhich will not be within the gamut of the printer and some of which maynot be physically realizable. i.e., within human perceptual gamut. (Thisis the case because the gamut of a set of colorants is not likely tohave a cuboidal shape suitable for representation in computer memorywhen it is converted from device coordinates to the coordinates of thecolor addressing scheme, as illustrated in FIG. 10B.) Therefore, thereis a favorable ratio of starting points to solutions.

An important advantage of keying off the FMT is that most searches areguaranteed to be for printable colors. For colors near the gamutsurface. Newton Raphson may fail because there is not an exact solutionsuch that an optimization procedure is required. The search routine mayuse either the interpolative approximation to the polynomial (the FMT)for speed in a software-bound system or the full-blown polynomial forgreater precision. In either case, the derivatives are easily calculableand the iterative searching procedures driven by gradients of colorerror (step 113).

The result of step 6 is a prototype color to colorant transformation(proto SEP) table in which virtually all printable addresses contain oneor more solutions. Because of the favorable ratio of starting points torequired solutions, many addresses will have solutions based ondiffering amounts of the neutral (black) colorant. The multiple blacksolutions are very useful in preparing to perform Gray ComponentReplacement and are stored in linked lists at the appropriate addressesof the proto SEP, which is stored in the local portion of the VP (step114). At this stage there are two ingredients of a final renderingtransformation in need of refinement: the prototype gamut descriptor andthe proto SEP.

An example of the results of step 6 is shown in FIG. 10B, in whichcoordinates which are plotted within a cuboidal structure suitable forrepresenting the perceptually uniform color space coordinates. Addresseshaving data are shown by black crosses. If the cube were subdivided intonumerous cuboids so as to create an interpolation table, as described inFIG. 9D, it is clear that many of the cuboids would not correspond tocolors that are realizable on the device. The coordinates of theperceptually uniform space are L*, u* and v* from the CIE's CIELUV colorspace in the example.

After step 6, the prototype GD data of step 5 is refined into finishedGD data at step 7 of FIG. 5. Step 7 processes are shown in the flowchart of FIG. 11. System 100 needs a favorable ratio of FMT entries toproto GD addresses; therefore, most of the addresses get one or moresolutions. Relatively few of these are surface Chroma points. Theobjective of step 7 is to use proto GD data as starting points for aniterative motion toward the gamut boundary using search and modelinversion techniques described previously. Gamut surfaces often manifestcusps, points and concave outward surfaces. Therefore, quantization ofthe descriptor must be at fairly high resolution (128×128 or more ispreferred) and initiation of searches for a surface point from othersurface points of different hue angle is often unfruitful. Because it isknown what colorant mix (i.e., little or no colorant) is appropriate atthe highlight white point of the gamut, it is best to begin refinementthere, using solutions at higher luminances as aids at lower luminances(step 121).

At least one of the colorants must be zero at the gamut surface.Therefore, the strategy is to start near the desired hue angle from wellwithin the gamut, move onto the desired hue angle and then drive outwarduntil one of the non-neutral colorants goes to zero (step 123). It isoften possible to zero the colorant that is expected to go to zero tohasten search procedures (such as Newton Raphson) which requireconstrained iterations. Starting points are usually available from alinked list stored in the proto GD; that failing, one may be had from aneighboring hue angle, lightness cell, or, that failing, by setting outfrom neutral in the desired hue direction (step 122). If polynomialevaluation hardware is available, it can accelerate on-the-flycalculation of surface points by forward model evaluation. Given a hueangle and lightness, the device coordinates bounding a small patch ofthe gamut surface are identified and colorant mixtures within the patchprocessed through the forward model until the maximum chroma at thehue/lightness coordinate is produced (step 124). In situations where itis necessary to fall back to methods involving model inversion, thehardware assist continues to be valuable because searching involvesnumerous evaluations of the polynomial and its derivatives, which arealso polynomials. Once a refined gamut descriptor has been completed, itis written to the shareable portion of the VP (step 125).

Referring to FIG. 12, a flow chart of the processes in Step 8 of FIG. 5is shown, filling in any holes which may exist in the PrototypeTransformation and computing functions that summarize the trade-off(Gray Component Replacement. GCR) between neutral and non-neutralcolorants at each color address. A “hole” is a color address which isprintable but for which no inking solution has yet been found. Theresulting proto SEP table is called a generalized color-to-coloranttable because it does not provide solutions for a specific amount ofblack. Step 8 has two goals. First, it seeks solutions for any colorsinterior to the gamut which have none (step 131) and writes NULL intoall unprintable addresses. The mapping of unprintable colors to withingamut occurs later in step 9 in response to gamut configuration data,which the user may select using the GUI screen of FIG. 21F, which isdescribed later. Second, it stores at each address the means to exchangeneutral for non-neutral colorants (Gray Component Replacement.) Thepreferred process is to store several solutions which can beinterpolated amongst using LaGrangian. Spline or generic polynomialinterpolating functions (step 132). The exact specification of blackutilization is incorporated later in step 9, which also may be selectedby the user via the GUI screen of FIG. 21E, which is also describedlater. The finished prototype color to colorant (proto-SEP)transformation table is written to the shareable portion of the VP (step133).

Step 9 of FIG. 5 includes the following processes:

-   -   1) Converting colorants of the proto-SEP transformation table        based on black utilization information specified in the black        color data;    -   2) Building Color to Color′ Transform Table based on gamut        configuration data. e.g., gamut scaling, neutral definition or        gamut filter(s); and    -   3) Combining Color to Color′ Transform Table with a        transformation table containing specific GCR solutions to        provide a rendering table.        The black color data and gamut configuration data may be set to        defaults or selected by the user as color preferences, as        described in more detail later.

Referring to FIG. 13, the processes of step 9 are flow charted. Althoughstep 9 is operative of four-colorant rendering devices, more-than-fourcolorant devices may also be provided with a rendering table as will beexplained in discussion of FIGS. 16A and 16B. First, the finished protoSEP table (generalized color-to-colorant) and the black utilization dataare read (step 140). The latter include Gray Component Replacement. %Under Color Removal (% UCR or UCR, a term referring to a limitation ontotal colorant application or Total Area Coverage. TAC) and possibleconstraints on the maximum amount of black or neutral colorant to beused; all three together are referred to herein as “black utilization.”

The prescriptions for black utilization come from one or more of thefollowing sources: a) local or system-wide defaults, b) broadcast ofcustom functions and limit values from a “boss node” configured innetwork 11 and c) values defined through a user interface which may beapplied locally or transmitted and shared. Note that modifications ofblack utilization do not have colorimetric effects. For example,compensating increases in neutral colorant by decreases in non-neutralcolorants in GCR does not change the color noticeably. Fields in the VPcontrol selection of the source of prescriptions for black utilizationunder particular rendering circumstances. A user may select the blackutilization in the Graphic User Interface of the model software, whichis described later in connection with FIG. 21E. The black utilizationdata, which includes % UCR, maximum black, and the GCR function or blacksolution, are stored in the shared part of the VP.

The first step in preparing a rendering transformation, then, is toconvert the data on GCR stored at each printable address into aparticular black solution by referring to the curve giving % GCR as afunction of density while observing the maximum black constraint (step149). Thus, the entries in the proto-SEP transformation table areconverted based on a specific GCR solution within a maximum black limit.The converted proto-SEP table is stored in the local part of the VP. Thesecond step is to find the maximum neutral density at which the totalarea coverage limitation is satisfied, i.e., % UCR (step 150). This isdone by moving up the neutral, or CIE Lightness, axis through thequantization scheme from the minimum printable Lightness, examiningcolorant solutions, until one is found that is within the limit. Theminimum Lightness so identified is stored for use in the gamut scalingprocess to be discussed presently. Although it is conceivable to storemin Lightness values as a function of color, rather than simply as afunction of Lightness, for purposes of gamut scaling, this is usually anunwarranted complexity.

In order to convert the GCR-specific result of the foregoing methodologyinto a rendering transformation, it must be “married” with a“conditioning” transformation. The latter is a color-to-color′conversion expressible as an interpolation table of the format presentedearlier. (Note that any transformation that is evaluated byinterpolation and that can be fitted with acceptable color accuracy by apolynomial may be performed by suitable hardware for polynomialevaluation.) It serves the multiple purposes of scaling unprintablecolors of the addressing scheme onto printable values. “aliasing” thecolor definition of neutral to accommodate user requirements (FIG. 21E,270) and effecting conversions among color addressing variables.

An example of the latter is the conversion from Cartesian CIELABaddressing to “Calibrated RGB” addressing which is useful if the imagedata that will be processed through a rendering transformation arerepresented as RGB. It will be described later that ConditioningTransformations (CTs) play an important role in verification and devicecontrol. A CT is often the result of the “concatenation” of severalindividual, conditioning transformations serving the purposes justidentified. The applications of transform concatenation include: 1) themethod whereby a separation table with many NULL entries is made usefulfor rendering by concatenation with a conditioning transform whichperforms gamut scaling with considerable savings in transform-generationtime and 2) feedback control of the color transformation process as willbe detailed later.

In order to minimize cumulative interpolation errors, intermediate colorto color′ transforms may be stored and/or evaluated at high precision.Wherever possible, exact conversions of table entries should beutilized. For example, once all the mappings of CIELAB color to CIELABcolor′ have been compiled, the conditioning table entries for a colorcoordinate conversion from CIELAB to “calibrated RGB.” for example, maybe computed using analytical expressions.

Note that system 100 does not preclude importation (as described indiscussion of User Interface) of standardly formatted colortransformations (“profiles”) prepared by applications other than theVirtual Proofing application. Accordingly, the rendering transformationmay incorporate user preferences indicated through atransformation-editing tool (called a “profile editor” by someApplication Software Packages.)

A key purpose of the conditioning transformation is gamut scaling,described below. It is important to elaborate this point: If the targetdevice for the rendering transformation is a proofer, then the outputgamut to which colors are scaled may be that of its client. Therelevant, conditioning data for all devices on the network resides inthe Virtual Proof. If the client's gamut fits entirely within theproofer's, then substitution of client for proofer gamuts is used torender a representation of how imagery will appear on the client. Thedisplay and negotiation of compromises to be made when the client'sgamut does not fit entirely within the proofer's is discussed inconjunction with FIG. 21F, below. Note that, in addition toaccommodating a client's gamut, successful proofing may require othermappings. For instance, tone scale remappings to compensate fordifferences in overall luminance levels between video displays andreflection media may be performed. Likewise. “chromatic adaptation”transformations to compensate for changes in illumination, viewingconditions. etc. may also be implemented by means of data structures ortables of what are called “conditioning” or color-to-color transforms(output color transforms) herein.

Elaboration of the concept of color “aliasing.” presented above is alsoappropriate: Neutrals are conventionally defined in terms of colorant inindustry: elements of the default neutral scale which appears in FIG.21E (step 270) generally do not have common chromaticity coordinates upand down the Lightness axis. In order to map colorimetrically neutraladdresses onto the desired mixtures of colorants, it is necessary toconvert the colorimetric addresses to the colors of the “colorantneutrals”—a process herein referred to as “aliasing” because one coloraddress masquerades as another. The term has a different usage, familiarto signal processing, when used herein in connection with imagingcolorimetry.

Specifically, the processes involved in making a color-to-color′transform (numbered 1 to 4 in FIG. 13) are as follows: (Recall that thegoal is to be sure that only printable address colors are applied to theGCR-specific SEP table in the course of making a Rendering Table.Normally, color out=color in at the outset. More detailed discussion ofinput gamuts, output gamuts and gamut operators follows. The sequence ofthe processes is important.)

1) Negotiate gamuts: In preparing a rendering transform to enable aproofing device to represent a printing press the color addressing ofthe color-to-color′ table may be defined by the input gamut—in terms ofthe color image data. The range of colors represented in the table islimited by the extreme image values in the three dimensions of UniformColor Coordinates. Because the quantization scheme is regular (cuboidal)there will be color addresses which do not occur in the image data(recall FIG. 10B.) These may be ignored, or mapped approximately ontothe surface of the image's gamut. In place of the image's gamut, ageneral representation of the gamut of the original medium of the imagemay be used.

This method is preferred, for instance, to using the gamut of the pressas the input gamut because so doing would require conversion of bulkyimage data into coordinates that are all within the press's gamut. Anobjective of system 100 is to provide means of interpreting image datain various ways without creating a multiplicity of versions of thatdata. An exception would occur if image data specifically from a presssheet Were required to evaluate image structure due to screening, etc.as part of the proofing process.

The output gamut may be the lesser of the client's (printing press) ANDproofer's gamuts, where “AND” connotes the Boolean operator whose outputis the “least common gamut.” This may be derived using gamut filters, asdiscussed later. In this negotiation, the proofer's gamut may beconstrained to be that available within the Total Area Coverage (% UCR)limitation applicable to the press at the user's discretion. Otherinteractive tools may be used to control the definition of the outputgamut subject to the ultimate restriction of what can be rendered on theproofing device within the default or selected % UCR constraint, ifapplicable. (Of course a video display proofer's gamut is notintrinsically subject to a UCR constraint.)

2) Perform gamut scaling: Using gamut operators to be discussed later,map color values to color′ and store the color′ values at the coloraddresses in the table.3) Perform neutral aliasing: In each lightness plane, the color of theconventionally neutral inking (FIG. 21E, 270) is offset from the neutralcolor coordinate a*=b*=0 by an amount a*,b*. In order to map imageneutrals to this offset color, the color′ values in the conditioningtable should be shifted in such a way that an address of 0,0 maps to acolor′ value of a*,b*. The function which performs the shift may bedesigned so that the amount of the shift decreases with distance fromneutral.4) Transform Color Coordinates (if necessary): The reason for this and amethod of implementation were suggested previously, namely, it ispreferred to perform gamut operations in Uniform Color Coordinates, butthe image data may be represented in a color notation such as“calibrated RGB” so that a rendering table must be addressable by RGBcoordinates. Because exact mathematical equations generally govern therelationship between Uniform CIE color and calibrated RGB, each color′value in the conditioning table is converted to RGB by means of theequations.

The Color-Color′ Transform (XForm) is produced by the above steps andthen concatenated with the GCR-specific SEP table. The result is arendering table for transforming the color of input color image datainto colorant data for the rendering device, which is stored in thelocal part of the VP.

The following discussion is related to providing gamut mapping data ofthe gamut configuration data. Color addresses that are not printablemust be mapped onto printable ones. In the present application, theoutput gamut is that of the proofer of interest or the printer itrepresents. However, the input gamut is not fixed by the architecture orsoftware design because it may vary and have a profound effect on therendering of out-of-gamut and nearly-out-of-gamut colors. This is thecase because the outermost, or limiting, colors vary greatly amongpossible input gamuts. Input gamuts that warrant distinctive processinginclude:

1) Other proofing devices include both hardcopy and video displaydevices (VDDs). Hardcopy devices are likely to have gamuts that arefairly similar, requiring only minor relative adjustments.Self-luminescent, additive-color devices such as video displays havevery differently shaped gamuts from reflection devices so that the bestmapping from input to output as a function of color will be unlike thatfor a reflection device. Computer-generated images originate inapplications which are likely to exploit the gamut of a video device.Many retouching and page assembly applications use the RGB coordinatesystem of the monitor to store and manipulate images because itfacilitates display on the VDD and interactivity. In this case, theinput gamut is often that of the VDD, even if the image was originallyscanned in from film.

2) Printing presses will usually have smaller gamuts than the proofingdevices that represent them, restricting what is to be used of theavailable proofing gamut. If printed images are captured by an imagingcolorimeter as part of calibration or verification so as to constitutethe image data, the input gamut may be better approximated by therendering gamut of the printer than the receptive gamut of thecolorimeter.

3) Electronic or digital cameras will usually have much greater gamutsthan any output device, necessitating a very significant restriction onwhat portions of the input gamut can be printed. Note, however, that themaximum gamut of a linear camera encompasses many colors that are notcommonly found in natural photographic scenes. Accordingly, it may bepreferable to design mapping functions for this class of device that arebased on the scenery as well as ones based on device capabilities.

4) In conventional photography, the scene is first captured on filmbefore being captured digitally. The relevant input gamut is therendering gamut of film.

5) Regardless of the input medium and its gamut, there may beimage-specific imperatives. For instance, a very “high-key” image,consisting almost entirely of highlights, lace, pastels, etc. may notmake extensive use of the available gamut of a color reversal film. Thebest gamut mapping for this particular image is not the generic one forfilm.

The gamut mapping data is provided by a gamut operator which is afunction which maps an input color to an output color. The process ofconstructing the gamut operator is shown in FIG. 14. It is a commonpractice to “clip” the input gamut to the output. In other words, allcolors outside the output gamut are mapped onto its surface. This may bedone in such a way as to preserve hue, saturation (Chroma) or Lightnessor a weighted combination of the three. System 100 can work with suchoperators and supports access to them through the “Rendering Intents”function in the GUI, as shown latter in FIG. 21F. However,invertibility, reciprocality and smoothness are preferred properties ofgamut operators, especially when processing and transforming image data.

Invertibility is an important property of the function because itinsures that no information is lost except that due to quantizationerror. Reciprocality means that a mapping of input color to output colormay involve either a compression or reciprocal expansion of the gamut,depending on which gamut is larger. Smoothness, or continuity of thefirst derivative, reduces the risk of noticeable transitions (“jaggies”)in images which are due to gamut scaling. A simple exemplar of aninvertible, reciprocal operator is illustrated in FIG. 14. It ispresented to demonstrate and clarify key concepts and then an operatorwhich is also smooth is explained. A mapping of Psychometric Lightness.L*, is shown, however, the same operator is applicable to CIE Chroma,C*, as well. It is assumed that hue is to be preserved in the mapping; aseparate tool is supported within the GUI to gamut operations latershown in FIG. 21F, for situations in which users want to modify hues.FIG. 14 depicts the two cases of reciprocality, one in which the dynamicrange of the input device must be compressed in order to fit the outputgamut and the other in which input lightnesses can be expanded to fillthe larger dynamic range of the output device. The operator introducedbelow is able to handle both cases without explicit user intervention,although operator override is also supported through the GUI of FIG.21F.

Define L_(pivot) as the greater (“lighter”) of the minimum input andoutput L* values.

L _(pivot)=max(L _(min) _(—) _(in) ,L _(min) _(—) _(out)),

where the minimum input lightness may, for example, be the L* value ofthe darkest color that can be reproduced on a positive reversal film andthe minimum output lightness the L* value of the darkest color printableon a reflection medium. L_(pivot) is denoted in FIG. 14A by a plaindashed line.

For lightnesses higher than L_(clip), the gamut operator maps input tooutput lightnesses identically as indicated by the equation in the openspace below the maximum L* of 100. A “cushion” is put between L_(pivot)and L_(clip) in order to insure that the mapping is invertible:

L _(clip) =L _(pivot)+(L* _(max) −L _(pivot))*cushion.

0.1 is a reasonable value for cushion, chosen so as to reduce the riskof losing information due to quantization error to an acceptable level.In the limit in which cushion=1, the entire range of input L* values isscaled uniformly, or linearly, onto the range of output L* values.

In either case 1 or 2, all lightnesses between L_(clip) and L_(min) _(—)_(in) are scaled onto the range of lightnesses between L_(clip) andL_(min) _(—) _(out), whether the scaling-represents a compression or anexpansion. A piecewise-linear scaling function is illustrated below forsimplicity. Note that all L values refer to CIE Psychometric Lightnessin these equations whether they appear with a * or not.

If ( L^(•) _(in) > L_(clip) ) then L^(•) _(out) = L^(•) _(in) Else   .L^(•) _(out) = L_(clip) − [(L_(clip)−L^(•) _(in))/(L_(clip)−L_(min) _(—)_(in)) * (L_(clip) − L_(min) _(—) _(out))] .

The concept can be extended by adding the property of smoothnessdescribed earlier. The operator is based on a continuouslydifferentiable function such as sine on the interval 0 to π/2, whichgenerally resembles the piecewise linear function described above inshape but has no slope discontinuity. A table (Table I) of values of thefunction is given below; the first column is a series of angles inradians, X, from 0 to π/2 (90°), the second, the sine of the angle, Y,and the third the fraction Y/X. If we set Y/X=(1-cushion), we cancontrol the “hardness” or abruptness of the gamut-mapping implemented bythe operator stated below the table in for the case of cushion˜=0.1. Forspeed, the various evaluations implied may be implemented byinterpolation in look-up tables. The operator described does not enablea purely proportional scaling (cushion=1.) The latter is not generallydesirable but is available to users through the gamut options of the GUIin FIG. 21F.

TABLE I Angle, x (rad) sin(x) sin(x)/x 0.0000 0.0000 * 0.0873 0.08720.9987 0.1745 0.1737 0.9949 0.2618 0.2588 0.9886 0.3491 0.3420 0.97980.4363 0.4226 0.9686 0.5236 0.5000 0.9549 0.6109 0.5736 0.9390 0.69810.6428 0.9207 0.7854 0.7071 0.9003 <------ “cushion” ≅ 0.10 0.87270.7660 0.8778 0.9599 0.8191 0.8533 1.0472 0.8660 0.8270 1.1344 0.90630.7989 If ( L^(•) _(min) _(—) _(in) < L^(•) _(min) _(—) _(out)) .707 * ((100 − L_(out)) / (100 − L_(min) _(—) _(out)) ) = sin [ .785 * ( (100 −L_(in)) / (100 − L_(min) _(—) _(in)) ) ] Else .785 * ( (100 − L_(out)) /(100 − L_(min) _(—) _(out)) ) = arcsin [.707 * ( (100 − L_(in)) / (100 −L_(min) _(—) _(in)) ) ]

The operators presented above rely on gamut descriptors to find thelimiting colors of the input gamut that must be mapped into the outputgamut. Once corresponding surface points in the two gamuts have beenidentified, the scaling function is used to prepare the conditioningtransformation.

In summary, input gamuts can be very different from output gamuts. Theyoften have a larger range of luminances (or dynamic range) such that itis preferable to perform a scaling in Lightness before working on Chromaor saturation. Secondly, scaling of Chroma is performed along lines ofconstant hue. Compensation for imperfections in the CIE models (embodiedin CIELAB and CIELUV) of hue constancy or for elective changes in hueare handled separately. An example of elective changes is the need tomap highly saturated yellows from film input to highly saturated printyellows by way of a change of hue angle. Modifications of hue can beaccomplished through the conditioning transformation by color aliasing.

Referring now to FIGS. 15A and 15B, the shareable and local componentsdescribed above in the VP are shown. In the interest of compactmessages, not all shareable data need be transmitted in each transactioninvolving Virtual Proof. To the application software which administersthe Graphical User Interface Software, both operating at a node, the VPis a set of data structures based around the classes Node andDevice/Transform. The data structures map onto a general file structurewhich is not bound to a particular operating system, computer platformor processor. Object-oriented conventions of data-hiding are employed inthe software to insure the integrity of transformations which aremanipulated at a node. Accordingly, the VP files which store the dataand transformations have local and shared components, as stated earlier;shared components consist of data which are read-only to all nodesexcept the one, assigned responsibility for the data. Duringinitialization in preparation for virtual proofing described inconnection with FIG. 18, participating nodes insure that appropriatedata are written to the relevant nodal fields within the shared filesystem.

The VP enables revision of color aspects of page/image data up to andeven during rendering. An important aspect of revisability is thecustomization of data for rendering on particular devices. An equallyimportant property of revisability is that page/image data need not bealtered directly; rather they are “interpreted” in various ways throughthe medium of the VP. This eliminates the need to maintain multiple,large versions of page/image data at a particular node or to move one ormore versions around the network repeatedly as a result ofre-interpretation. Therefore, although the VP allows for imagespecificity and linking, preferably it is not bound into page/imagefiles. The structure of the VP is similar to that of the Tagged ImageFile Format (an example is described in “TIFF™ Revision 6.0”, Jun. 3,1992, Aldus Corp., Seattle Wash., pp. 13-16).

An example of the tagged or linked list file format for the shared partsof the VP is shown in FIG. 15C. The tags are referred to as Field IDs(bottom right of FIG. 15C.) It is possible for a given device to bepresent in a VP file more than once, specialized to represent images fordifferent input gamuts or black utilization, etc.

System 100 may incorporate rendering devices at nodes having more thanfour colorants. The processes for performing color to colorantconversions for more than four colorants is shown in FIGS. 16A and 16B,which are connected at circled letters A and B. In FIG. 16A, afterstarting, if the extra colorant is neutral, then proceeding continues tothe start of FIG. 16B. Otherwise the following steps for addingadditional non-neutral colorants (e.g., Red, Green and Blue) areperformed:

Step 1) Proceed as for 4 inks through to the stage of gamut scaling. Usethe Black Utilization tool which enables % GCR to depend on Chroma, topush towards maximum Black (2-colors+Black, with the complementary colorpushed toward zero) solutions. Save this as an intermediate table. Thisintermediate is the equivalent of a GCR-specific SEP table.

Step 2) Build a model for converting C. M Blue and K (black or N) tocolor and omitting the colorant complementary to the “auxiliary.” inthis case. Yellow. Make a Forward Model Table and use the model toextend the original gamut descriptor prepared in (a). Do likewise for C.Y. Green and K and M. Y. Red and K. Note that the general rule is to addadditional colorants one at a time, grouping each with the colorantswhich flank it in hue angle. Make FMTs for each new model for eachauxiliary colorant and re-refine the Gamut Descriptor. Note, however,that the multiple models are used to refine only one GD.

Step 3) Modify the proto-rendering table (only one of these ismaintained): Within the C.M.Blue, K gamut, there are not multiplesolutions with different amounts of black at a given color; however,there are many solutions trading off CM vs. Blue. Store linked lists ofthese solutions at relevant color addresses in the intermediate table.Do likewise for CYGreenK and MYRedK.

Step 4) Treat the intermediate table as a “prototype table” in the4-colorant case. Perfect it by making sure that all printable addressesin the new color regions of the table have at least one solution (“fillthe holes.”)

Step 5) Once the intermediate table has been reprocessed for allnon-neutral auxiliary colorants, convert to a rendering table byperforming the analog of GCR in the three new regions of the table.Check total area coverage, re-scale gamut and complete as for four inks(Step 9, FIG. 5.)

The foregoing procedure does not estimate the full gamut available; forexample, the gamut at the hue angle of cyan is increased by theavailability of blue and green. In other words, the BCGN gamut (where Nstands for Neutral, or black) is not considered in the foregoing.Overprints of Blue and Green are likely to be substantially dark,compared to cyan. Therefore, the additional colors available in thisgamut are not, in general, very numerous and need not be computed.However, in those cases in which the lost colors are important, theprocedure outlined above is extended to include auxiliary gamutscentered on the standard subtractive primaries (C, M and Y) rather thanon the additional colorants (R. G and B.) The result is overlappingauxiliary gamuts. By default, the decision regarding which of theoverlapping auxiliary gamuts to search for a solution chooses the gamutcentered on the ink of closest hue angle. There is nothing in theprocedure which prevents its extension to even more colorants, which maybe added in a recursive fashion. However, practical applicationsinvolving the overprinting of more than 7 (or 8, in the case of an extraneutral) colorants are very unlikely.

After the above steps are complete, if more colorants need to be added,processing branches to the circle A at the start of FIG. 16A, otherwisethe process for adding additional colorants is complete. In the case ofaddition of auxiliary colorants which does not involve overprinting morethan 3 or 4 colorants at a time (as in the case of multiple, “custom”colorants that might be used in package printing) the colorants aretreated as separate sets according the procedures outlined previously.

If the process branched to FIG. 16B (to circle B), then the followingsteps for adding an approximately neutral colorant, such as gray, areperformed: If additional non-neutral colorants are also to be added, addthem according the procedure outlined in FIG. 16A above.

Step a) Prepare a colorant to color transformation for the 5-colorantset CMYKGray. Evaluate this model either with polynomial hardware orwith a 9×9×9×9×9 interpolation table having about 60,000five-dimensional cells. The simple linear interpolator is preferred andis particularly appropriate to this situation because the complexity ofcalculations scales linearly with the dimensionality of theinterpolation space. As usual, make a Gamut Descriptor in tandem withbuilding FMT.

Step b) Invert the model as in the 4-colorant case, fixing black andgray; build up linked lists of alternative solutions for a given color.

Step c) Proceed as in the 4-colorant case. When inverting the model, useonly CMY and Gray wherever possible (i.e., fix black at zero,) addingblack only as necessary to achieve density. There are two stages of GCR.In the first, black is held to a minimum and gray is exchanged with C, Mand Y. In the second, black may be exchanged, optionally, with gray andsmall amounts of the other colorants, as needed to keep the colorconstant. In the second stage, an error minimization routine is needed;Newton-Raphson is not appropriate.

Step d) UCR, preparation of a conditioning transformation, and so on asin the 4-colorant case, follow the second stage of GCR. Complete therendering table, except for addition of auxiliary, non-neutralcolorants.

After the above steps a-d, if additional non-neutral colorants are alsoto be added, processing branches to circled A in FIG. 16A, otherwise theprocess for adding additional colorants ends.

Referring to FIG. 17, the process for building a gamut filter is shown.The finished prototype color to colorant table, filled with NULL cellsand specific GCR solutions, is one manifestation of a device's gamut. Itcan be converted into a very useful filter in which each cell gets anindicator bit. 0 if the cell is NULL and 1 otherwise. The filters of twoor more devices can be used to enhance visualizations.

Many graphics cards for driving video displays provide an alpha channelor overlay plane enabling the visualization of translucent graphicalinformation on top of a displayed image. The results of performingBoolean operations on combinations of gamut filters for multiple devicesmay be converted into color-coded or pseudocolor overlay information toreveal things such as which image pixels are in gamut for one device butnot another. With this tool, the intersection of the gamuts (the “LeastCommon Gamut”) of five presses can readily be compared with each press'sgamut in turn on common imagery, in support of a decision about what touse as a common criterion for color reproduction across the network.

Semi-transparent overlays are generally not possible for hard-copydevices without extensive image processing. In the case of a printer, amonochrome version of the image may be prepared and printed, overlaidwith colored speckles indicating regions of overlap of multiple gamuts.The “overlay” is actually constituted by redefining subsets of thepixels in the image which belong to a certain gamut category as aparticular speckle color. The foregoing method involves making amodified copy of the color image data with reference to the gamutfilter.

An alternative, and preferred, method provided by the invention forreadily identifying gamut overlaps is to filter the color-to-color′transform or actual rendering table. This may be done by leaving thecontents of “in” addresses as they were while modifying the color orcolorant contents of one or more varieties of “out” addresses to containwhite or black or other identifying color. Different identifying colorsmay code different regions of intersection, overlap or disjointedness ofthe gamuts of several devices. When one or more channels of the(original) color image data are rendered through the “filtered renderingtable.” colors in the image which are out of gamut are mapped to one ofthe identifying colors and the resulting print reveals gamut limitationsof various devices with respect to the imagery itself. An additionaladvantage of this method is that it is effective even when some of thecolors under consideration are out of gamut for local proofing devices.

Another method of visualization available with the filters is to look atslices through Boolean combinations of the filters for two or moredevices. Finished proto SEP tables are not generally useful other thanin the preparation of rendering tables for a particular device;therefore, they are kept local. The filters are written to the sharedportion of the VP.

More specifically, in FIG. 17 a flowchart of the process of making agamut filter in either a compressed form, in which a single bit is 0 or1 to denote NULL or “in-gamut.” or in a form in which each color'sstatus is coded with a byte equal to 0 or 1 is illustrated. In thecompressed case, a computer word may be used to represent a row or lineof the prototype color-to-colorant table with each bit packed in theword representing one color address. After reading the proto-table (step1,) the steps of making the compressed filter include 2c) looping overthe lines of the table, 3c) setting or resetting the bits of a worddepending on printability and 4) finally writing the filter to shareableVP. The steps for making a byte table are completely analogous, exceptthat a byte is devoted to each color address.

The two basic processes of calibration and verification (monitoring)rely on instrumentation and interact to produce the rendering transformwhich embodies the VP and can interpret color image data.

Referring now to FIGS. 18A-B, a flow chart of the program operating atnodes 102 and 104 for virtual proofing in system 100 is shown. Thefigures are connected at circled letters A-D. At the top of FIG. 18A,one or more users invoke the application software 1801; a single usercan run the program in order to revise the Virtual Proof as it relatesto the local node, or to other nodes which are accessible. Often,multiple users run the program simultaneously to negotiate the VP.Network readiness is established by making sure that the relevant,participating nodes all share current data 1802. CMI's are put through(preferably automatic) calibration checks 1803. Next, verification ofdevice calibration is attempted by rendering and analyzing color 1804.The details of this step depend on the nature of the device and of thecolor measurement instrument: if the device is a press, particularly onewith fixed information on the plate, the verification is most likely toinvolve “live” imagery rather than a predefined cal/verification form ortarget such as one consisting of tint blocks. The color error data 1805produced by verification are supplied to the press controls, ifappropriate 1806, as well as to the program to support decisions aboutwhether color variations can be compensated by modification of existingrendering transforms of the VP or whether recalibration of the device isrequired 1807.

If re-calibration is called for, the program branches to C, atop FIG.18B, where systematic calibration after FIG. 5 is undertaken 1808. Else,the program branches to B, atop FIG. 18B, to revise the color-to-color′transform 1809 based on the processing of the color error data, which isdetailed later in FIGS. 19 and 20. Next, the need for User-preferencerevisions is assessed at D. If YES, then gather User preference data1810 and re-specify rendering transforms 1811 as in Step 9, FIG. 5. IfNO, then revise VP for a new interpretation of image data and render1812. If results are satisfactory, conclude. Else, either recalibrate atA or revise preferences at D, depending on the nature of thediagnostics.

Verification is a feature of system 100 used in virtual proofing,described above, and color quality control of production renderingdevices. The reason for verification is that the use of system 100 forremote proofing and distributed control of color must engenderconfidence in users that a proof produced at one location lookssubstantially the same as one produced in another location, providedthat the colors attainable by the devices are not very different. Oncerendering devices are calibrated and such calibration is verified toeach user allowing, virtual proofing can be performed by the users atthe rendering devices. In production control, such verification providesthe user reports as to status of the color quality.

During verification of production rendering devices, on-press analysisof printed image area may be used in control of the production processand for accumulation of historical data on color reproductionperformance. Historical data may be used in a statistical profile of themanufacturing run which serves as a means of verifying the devicecalibration. It is also used to inform and update the virtual proof,enabling better representation of the production equipment by a proofingdevice. With a sufficient accumulation of historical information, it iseven possible to model and predict the effects of neighboring pages in asignature on the color in the page of interest to an advertiser.

Once a device has been calibrated, the color transformations thus can beone of the mechanisms of control. In a control loop, colors produced bythe device are compared to desired values and mechanisms affectingcolorant application are modulated to reduce the discrepancy betweenmeasured and desired values. Control implies continuous feedback anditerative adjustment over many printing impressions, whereas proofingdevices are usually one off. Nevertheless, proofing devices vary withtime and periodic recalibration is a means of control.

One feature of system 100 is to provide the User with information aboutthe color accuracy of the proof. It has been noted that the invention iscompatible with two kinds of instrumentation, unitary colorimeters (SOMs13) capable of measuring homogeneous samples and imaging colorimeters(imagicals 14) capable of sensing many pixels of complex image datasimultaneously. In the following discussion, differences in verificationprocedures for the two kinds of instrument are considered.

Calibration is best carried out with a specialized form which is knownto explore the entire gamut of the device. The rendered form can bemeasured by either type of instrument. In verification, the requirementsof sampling the entire gamut are not as stringent; the interest is oftenin knowing how well the reproduction of all the colors in a particularimage is performed, even if the image samples only a part of the devicegamut.

Referring to FIG. 19, steps 1804-1807 of FIG. 18A are shown. Aspecialized verification image is analyzed either with a SOM 13 or animagical 14 according to the following procedures: Step 1: Render animage consisting of homogeneous samples (“patches”) of three types: a)patches whose nominal colorant specifications match those of patches inthe original calibration, b) patches whose nominal specs are differentand c) patches specified as color. The original calibration procedure(see FIG. 8) produced statistical estimates of within-sheet andbetween-sheet color differences or process variation. User-definedrequirements for accuracy are expressed in terms of standard deviations(or like quantity) within the process variation to define confidencelimits for the process. Three kinds of color error derived fromverification procedures are used to control the process and are referredto the confidence interval in order to decide if recalibration isnecessary.

Step 2: New measurements of type “a” patches (preceding paragraph) arecompared to historical values to estimate change in the process; athorough sampling of color space is useful because it is possible thatchange is not uniform with color. Step 3: Model predictions of thecolors of patches of types “a” and “b” are compared to measurements toyield estimates of the maximum and average values of color error for theforward model. Step 4: Comparisons of the requested colors of patches oftype “c” to those obtained (measured) are used to estimate the overallerror (due to change of the process, model error, model inversion error,interpolation and quantization errors, when relevant.) Step 5: If colorerrors assessed in this way exceed the confidence limits, the User(s)are advised that the system needs recalibration and corrective actionsmay also be taken, such as modification of conditioning transforms,depending on the severity of the problem. If the device is a press,color error data is furnished to the press control system (subject toUser intervention) which does its best to bring the press as close tocriterion as possible.

The advantage of specialized imagery is that suitably chosen patchesprovide more information than may be available from imaging colorimetryof images with arbitrary color content. This can guarantee that theentire gamut is adequately sampled and can provide differentiatedinformation about sources of error. Also, imaging colorimetry of thereproduction during or shortly after rendering is often the leastobtrusive way of verifying that forward models and renderingtransformations are current, especially when a volume production deviceis the object.

Because, as was noted above, the variations in color need not be uniformthroughout the gamut of the device, the data structure is segmented intoclusters of contiguous cells in order to identify the most frequentcolors in the various regions of the gamut. Thus, system 100 hereinsamples color errors throughout the image is one of the points. Theprocessing checks to make sure that the frequency it is reporting for acluster of cells is a peak, not a slope from a neighboring cluster.

In order to improve the reliability with which corresponding peaks indifferent histograms can be resolved, methods of image processing suchas accumulation of counts from multiple images (averaging,) bandpassfiltering and thresholding are employed. Then regions of the histogramsare cross-correlated. Cross-correlation is a technique discussed in manytexts of signal and image processing, in which two functions areconvolved without reflection of one of the two. It is similar totechniques in the literature of W. K. Pratt, Digital Image Processing,NY: Wiley, 1978, ch. 19, pp. 551-558. A “cross-correlogram” reveals theoffsets of one histogram with respect to another in three-space.

The color offsets of the peaks are expressed as color errors. These aremade available for numerical printout as well as for visualization. Inthe latter, the user may choose to view a monochrome version of theimage overlaid with translucent color vectors showing the direction andmagnitude of color-errors, or may choose to view a simulation of theerror in a split screen, full color rendering of two versions of theimage data showing what the error can be expected to look like.

For clarity, an equivalent procedure for cross-correlation can beoutlined as follows: 1) subdivide the histograms into blocks and“window” them appropriately, 2) calculate Fourier Transforms of thehistograms, 3) multiply one by the complex conjugate of the other, 4)Inverse Fourier Transform the product from 3 and 5) locate the maximumvalue to find the shift in the subregion of color space represented bythe block.

For the simplest level of control, the inverse of the color errors maybe used to prepare a conditioning transformation which then modifies therendering transformation employed in making another proof. For moresophisticated, on-line control, the data are used to compute errorgradients of the sort described earlier and used by optimization anderror minimization algorithms. Results are fed to the control processorof a press or used to modify the rendering transform as a controlmechanism for a press (or press-plate) which does not use a pressbearing fixed information.

The goal is to determine errors in color reproduction in aresolution-independent way. This is shown in reference to FIG. 20,illustrating processes 1804-1807 in FIG. 18A when an imagical 14 isverifying using live image data. In step 1 of FIG. 20, the histogram isdefined. Generally, it is a data structure addressed similarly to theConditioning Transform (color-to-color′ table) described earlier,although the range of colors represented in each dimension of thestructure is adaptive and may depend on the imagery. In step 2, the 3-Darrays toehold accumulated histogram data are allocated in memory andinitialized; one array is needed for “live” image data and the other forreference data. At step 3, capture of “live” image data occurs. Opticallow-pass filtering may precede image capture, preferably by a solidstate electronic sensor, in order to reduce aliasing in signalprocessing. The electronic, pixel data are converted into CIEcoordinates, and, simultaneously, a histogram of the relative frequencyof occurrence of colors in the image is stored. As mentioned earlier,the data structure may be segmented into clusters of contiguous cells inorder to identify the most frequent colors in the various regions of thegamut.

In part 4 of the process, the image data (not that captured by theimagical, but the “original,” image data) are processed through thecolor-to-color′ transform to produce Reference Color Data which areaccumulated in a histogram structure. It is important to recognize whatthe requested (or “reference”) colors are. They are the colors(preferably in CIE Uniform coordinates) which are the final outputs ofall the color-to-color′ conditioning transforms (usually exclusive ofcolor-coordinate transforms) and thus represent the interpretation ofthe image data negotiated by the Users.

In steps 5 and 6, as described above, the program checks to make surethat the frequency it is reporting for a cluster of cells is a peak, nota slope from a neighboring cluster. Accumulation of counts from multipleimages (averaging,) bandpass filtering of histograms, thresholding,autocorrelation and other operations of image processing are used toimprove reliability and the ability to resolve peaks and matchcorresponding peaks in different histograms. Ordered lists of peaks areprepared and written to the shareable portion of the VP. The lists arecompared and corresponding peaks identified. The color offsets of thepeaks are expressed as color errors. In step 7, color error data aremade available to User/Operator and control systems.

Referring to FIGS. 21A-21F, a Graphical User Interface (GUI) to theapplication software is shown. The GUI is part of the software operatingat the nodes in network 11 for conveying the workings of system 100 at ahigh-level to the user. The user-interface has reusable softwarecomponents (i.e., objects) that can be configured by users in apoint-and-click interface to suit their workflow using establishedvisual programming techniques. The GUI has three functions: 1) Network(configure and access resources,) 2) Define (Transformation) and 3)Apply (Transformation.) All three interact. For instance, verificationfunctions fit logically within Apply Transformation but must be able tofeed back corrective prescriptions to Define Transformation whichprovides a superstructure for modules concerned with calibration andmodelling. Therefore, both Define and Apply need access to ColorMeasurement modules, whether they make use of imaging or non-imaginginstruments. “Network” is responsible for coordinating network protocolsand polling remote nodes. Part of this function includes theidentification of color measurement capabilities of a node. Another partis to insure that a user's mapping of software configuration onto hisworkflow is realizable. Loading the appropriate color measurement devicedrivers is as crucial as choosing and initializing the correctcommunications protocol and proofer device drivers. Therefore, ColorMeasurement coexists with modules for administering network protocols,building device models, building color transformations and implementingthe transformations for the conversion of color images.

For the purposes of this discussion, assume that the application isstand alone. Today, Graphical User Interfaces can be prepared viaautomatic code generation based upon re-usable components in most of thewindowing environments on the market. The depictions of menus andattributes of the User Interface in what follows are not meant torestrict the scope of the invention and are kept simple for clarity.

Referring to FIG. 21A, a first level hierarchy screen is shown allowinga user to enable configuration of network 11 nodes, remote conferencing,and user oversight of processes in system 100. Upon invocation, theapplication introduces itself and offers a typical menu bar with fivechoices (command names). File. Network. Define, Apply and Help. ClickingFile opens a pull-down menu whose selections are like those indicated by221 in the FIG. Clicking on Create 222 initializes file creationmachinery and opens the Network tableau (FIG. 21B) in a mode ofreadiness to design a virtual proofing network. Export VP (VirtualProof) 223 offers the option of converting color transformationalcomponents of the Virtual Proof into standardized file formats such asInternational Color Consortium Profiles. Adobe Photoshop colorconversion tables, PostScript Color Rendering Dictionaries. TIFF orTIFFIT. A possibility of importing standardized color transformationfiles is also provided. Other menu items under File are conventional.

The Network heading 224 opens a tableau concerned with membership in thevirtual proofing network, the physical and logical connections amongnodes and the equipment at the nodes and its capabilities. The Defineheading 225 provides means of calibrating (characterizing) devices,enabling customized assemblies of procedures to be applied toappropriate target combinations of devices and color measurementinstrumentation. The Apply heading 226 covers display of imageryrendered through the virtual proof and provides tools for verifying andreporting on the accuracy of color transformations, customizing thosetransformations, conferencing, comparing various versions of the proofand establishing contact with other applications performing likefunctions. The Main menu offers Help and each of the Network, Define andApply menus offer Tutorial interaction.

Clicking on Network in the Main menu opens a tableau of FIG. 21B, whichis concerned with Connections and Capabilities. Clicking Connection 227reveals a sidebar concerned with tools and attributes of network 11connections. For example, during creation (see FIG. 1,) it is possibleto pick up a wire by way of the “wiring” entry of the sidebar 228 andmove it into the field as part of assembling a network model that can bewritten to a file and can direct proofing commerce. Double clicking onthe wire reveals information about the connection or permits editing ofthe information when the “Modify” radio button 229 is activated. Errornotices are generated when the software drivers required to implementthe model are not available or when the hardware they require is notpresent. The present invention is not wedded to particular current(e.g., modem, ISDN, T1, satellite, SMDS) or anticipated (ATM)telecommunications technologies. nor to particular networking protocols.Nodes can be defined, given addresses. security, etc. and can beequipped with proofing devices in a manner similar to the design ofconnections. The summary of the network's connections and of nodalcapabilities is shared through the Virtual Proof's tagged file formatdescribed earlier which is kept current at all sites.

In the center of FIG. 21B is an example network topology. It resemblesthe network of FIG. 1, where “cli” 230 refers to a possible client(e.g., advertiser) member. “ad” 231 an ad agency, “pub” 232 a publisher,“eng” 233 an engraver and the Ps are printers. The links between thenodes are created and modified through the wiring 228 functionality of“Connection” mentioned above. Clicking “Capability” reveals a sidebar234 concerned with devices and their associated instrumentation forcolor calibration and verification. An example of the use of the menu isas follows: I am a user at an ad agency who wants to establish a remoteproofing conference regarding a page of advertising with a publisher andprinter. I bring up the relevant network using “Modify . . . ” in FIG.21A, push the radio button for view/select 235 in FIG. 21B, and click onthe “pub” node 232. This creates a connection between the ad and pubnodes if one can be made and initiates a process of updating virtualproof files at either end of the link. Then I click on hard proof 236and color measure 237. This utilizes the updated VP information to showme what hard copy proofer(s) are available and how they are calibratedand/or verified. Then I follow a similar sequence of actions withrespect to a P node. The initiation of display and conferencing aboutcolor image data is done via the Apply menu of FIG. 21D.

To pursue the example of the preceding paragraph, suppose that I findthat the hard copy proofer at node “pub” has not been calibratedrecently. A study of the information about the device in the updatedVirtual Proof reveals whether re-calibration or verification procedurescan be carried out without operator intervention at that site. From onesite or the other, the Define (Transformation) menu of FIG. 21C providesaccess to the tools and procedures for calibrating while the Apply(Transformation) menu (FIG. 21D) supports verification. A node can beactivated in the Network menu and then a device at the node singled outfor calibration within Define.

Clicking on “Node” 241 in the bar atop the Define menu of FIG. 21C opensa pull down providing access to other nodes without requiring a returnto the Network menu. Which node is active is indicated at the upper leftof the menu 242. Clicking on “Devices” 243 opens a pull-down which listsclasses of devices; clicking on a member of that list displays aninventory of devices of that class present at the active node. Selectinga device in this list is the first step in the process of calibrationand causes the device to be identified as active 248 at the top of themenu. The classes of devices of particular interest in the invention areimaging colorimeters or imagicals 14 (“imagicals.” 244,) unitarycolorimeters (SOMs 13, capable of measuring discrete tint samples, 245,)presses 246 and proofers 247 of hard and soft copy varieties. Clickingon “Procedures” 249 reveals a pull down listing calibration modules 250such as linearization, Forward Model Generation, etc.

Procedures appropriate to the device can be dragged and dropped into theopen field at the center of the menu and linked by connecting arrows.The controlling application software monitors the selections, performserror-checking (with notification and invocation of tutorial material)and links together the modules needed to perform the task if possible.FIG. 21C shows a flowchart for complete calibration of a renderingdevice encircled by a dotted line 251. In the case of some proofingdevices, such as Cathode Ray Tube displays and Dye Sublimation printers,it may be sufficient to perform complete calibration only infrequently.In particular, it is usually adequate to re-compensate for the gammafunction of a monitor (a process which falls under “linearization”) on aregular basis. Because phosphor chromaticities change very gradually,re-determination of the color mixing functions of the calibration neednot be performed as frequently. Therefore, a user may activate the CRTdevice at his node and specify only linearization in preparation for aproofing conference. Alternatively, the present invention covers theequipment of monitors with devices that can participate in continualverification and recalibration.

The “Apply” Transformation menu (FIG. 21D) provides access to thedatabase of pages and images that are the subject of remote proofingthrough the “Page/Image” 256 selection in the menu bar. Clicking heredisplays a shared file structure. Although the (generally) bulky colorimage data file of interest need not be present in local storage 19 ofFIG. 3A at all nodes where proofing is to occur, it is generallydesirable to make a local copy so that rendering across the network isnot necessary. However, one of the purposes of the Virtual Proof is tomake multiple transfers of the bulky data unnecessary. The “Node” 257and “Devices” 258 elements of the menu bar have effects that areentirely analogous to those in the “Define” menu of FIG. 21C. More thanone device at a node can be made active in support of a mode in whichinteractive annotation and conferencing via the medium of a soft proofon a video display is employed to negotiate changes in a hardcopy,remote proof that is taken to be representative of the ultimate clientdevice.

Clicking on “Procedures” 259 in the Apply menu bar of FIG. 21D reveals apull down that includes functions such as “Render to display . . . ”260. “Verify . . . ” 261 and “Window . . . ” 262 to externalapplications. Rendering supports display, either within the Apply windowor on a separate, dedicated video display, of imagery a) as the designerimagined it, to the extent that the proofer is capable of showing it, b)as a client device, such as a press, can reproduce it and c) as anotherproofer is capable of representing the press, including indications ofgamut mismatches superimposed on the imagery and location of errorsidentified by the verification process. To further the example, thevirtual proof may mediate a rendering of an image as it appears or willappear on press. If the node includes an imaging colorimeter, then animage of the proof can be captured and analyzed in order to provideverification of how well the proofer represents the client. Withoutverification, digital proofing and remote proofing for color approvalare not really useful.

The Apply menu provides a Window 262 through which to “plug-in” to or to“Xtend” applications such as Adobe Photoshop or Quark Xpress. It alsoprovides the platform for incorporating remote, interactive annotationof the sort provided by Group Logic's imagexpo, reviewed earlier.Imagexpo focusses on marking up images with a virtual grease pencil, aconcept that is extended in the present invention to remote conferencingconcerning gamut scaling, black utilization and other aspects of thedefinition of rendering transforms. Aspects of rendering such as blackutilization 263 (or gamut operations) can be harmonized across theproduction network by sharing/exchanging black designs through thevirtual proof file structure and mechanism.

Menus supporting interactive design and selection of user preferencedata are shown in FIGS. 21E and 21F. A User-Interface to supportinteractive determination of black utilization is depicted in FIG. 21E.It may be invoked from either Define or Apply menus. At the top right270 is shown a panel of colorant specifications which are to beconsidered neutral in color. Users may redefine entries in the panel byclicking and keying or by modifying curves in the graph below 272provided the graph is toggled to neutral definition rather than GCR. Inthe neutral definition mode the user may move points on any of thecolorant functions; the points are splined together and changes in thegraph are reciprocal with changes in the panel. A “return to default”switch 274 provides an easy way to get out of trouble. At the upperright 276, 278, variable readout switches enable definition of maximumcolorant coverage and maximum black. At the bottom right 280, “CustomizeTonal Transfer” opens the door to user modification of one or more ofthe 1-dimensional output postconditioning LUTs which are part of thecolor to colorant transformation. The application warns sternly that thespecification of transfer curves which did not prevail duringcalibration will void the calibration; however, there are situations inwhich knowledgeable users can make effective use of the flexibilityafforded by this function.

When the Graph is switched 282 to GCR mode, the user can control onlythe shape of the neutral colorant curve; because GCR is colorimetric,the non-neutral curves respond compensatorily to changes in black. Therelationship of the curve specified in the graph to the amount of blackchosen for a solution at a particular entry in the separation table isas follows: The function shown in the graph represents amounts of thecolorants which produce given neutral densities. At each density, thereis a range of possible black solutions from minimum to maximum. At theminimum, black is zero or one or more of the non-neutral colorants ismaxed out; at the maximum, black is at its limit and/or one or morenon-neutral colorants are at their minimum. In this invention, the % GCRis the percentage of the difference between min and max black chosen fora solution. By industry convention, a constant % GCR is seldom desiredfor all Lightness (density) levels. Therefore, the black curve in thegraph defines the % GCR desired as a function of density. Although it isconceivable to make GCR a function of hue angle and Chroma as well as ofLightness, this is usually an unwarranted complexity with one exception:It is useful to graduate GCR with Chroma when preparing transformationsfor more-than-four colorants as discussed earlier. This need isaddressed through the “GCR special . . . ” submenu 284 offered at thebottom right of FIG. 21E.

FIG. 21F depicts a GUI screen to gamut operations. Clicking on “Gamuts”reveals a pull-down 286 which provides access to lists of input, outputand client gamuts; the latter two are both output gamuts, but a clientgamut is known to the system as one that is to be represented on anotherdevice. It is possible to drag and drop members from the different typesinto the field of the menu and to link them with various procedures, aswas the case in FIG. 21C. Procedures applicable to the gamutsinclude: 1) Analyze 288 which displays information on how a particularconditioning transformation was put together (e.g., was it aconcatenation of gamut scaling and color aliasing operations?—whichones?) and on gamut compression records, a constituent of the VP whichstores key variables of a gamut scaling, such as minimum Lightness,cushion value, functional form, etc. 2) Apply to Imagery 290 enablesdisplay of imagery mediated by transformations configured in this orrelated menus on some device. 3) Compare Gamuts 292 enablesvisualization, in several forms, of the relationship between the gamutsof two or more devices—this functionality is elaborated in a followingparagraph. 4) Concatenate 294 does not apply solely to gamut operations;it links nested or sequential transformations into implicit, nettransformations. 5) Gamut Operator 296 provides a graphical display ofan operator; this is a different representation of the informationavailable from Analyze 288. 6) Negotiate Proofing Relationship 298 worksclosely with Compare Gamuts 292; it enables users to make decisionsbased on information provided by Compare, such as whether to use theLeast Common Gamut as the aim point for a network of volume productiondevices. 7) Remap Hues 300 provides the separable hue adjustmentfunctionality described earlier. 8) Rendering intents 302 is a mechanismfor providing users with generic gamut scaling options for accomplishingthings like obtaining the most saturated rendering on paper of colororiginally created in a business graphics application on a videodisplay. Compare Gamuts 292 allows a user to invoke and control the useof gamut filters, which were described earlier in connection with FIG.17.

System 100 supports the coordination of color control in multisiteproduction in several forms. Because the virtual proof encompassesrecords of the states of calibration of network devices independent ofthe color data, application software can define a criterion forreproduction across the network in one of several ways. Based on thestates and capabilities of network devices, a criterion may be selectedwhich all devices can satisfy, such as a least common gamut (LCG). LCGdefines the aim point for production and the control system strives tominimize the color error with respect to it. Alternatively, users mayselect one device as the criterion and all others are driven by thecontrols to match it as closely as possible. Optionally, users maychoose to disqualify one or more rendering devices on the networkbecause it/they cannot match the criterion closely enough or due tofailures of verification.

Referring to FIG. 22, an example of system 100 (FIG. 3A) is shown havingtwo nodes of the network. Node N may possess high performance computingprocessor(s) and, optionally, extensive electronic storage facilities.Node N may also have output devices of various types along with colormeasurement instrumentation for the calibration of those devices and itmay be connected to more than one networks for Virtual Proofing. Inaddition to participating in one or more networks for Virtual Proofing.Node N may assist other nodes by computing transformation functions. Itmay also function as a diagnostic and service center for the networks itsupports.

Node A 310 includes components similar to nodes 102 or 104 (FIG. 3A) andis linked to other nodes by network link 11 a. The communication betweenNode A and Node N is enabled via the Internet or World Wide Web, forexample, to a web site service “cyberchrome” 315 at Node N 312. Thiscommunication is illustrated by the screen of the video display device311 being labeled “www.cyberchome”. Node A 310 has a computer 314 suchas a personal computer or workstation in accordance with applicationsoftware providing Virtual Proofing as described earlier. Node A 310need not have a computer other than the processors embedded in theproofing devices or color measurement instrumentation. In this case, theVirtual Proof for Node A are coordinated by another computer system,such as the computer server 316 at Node N (called hereinafter theprofile server), as described earlier in connection with FIG. 3A.

Printer 317 is shown for Node A, but not for Node N. A color measurementinstrument (CMI) 318 is provided as a module for calibration of theprinter. CMI 318 includes a sensor, lamp, reference and control unit(which may itself be of modular design) and a transport mechanism 320for transporting hard copy of a calibration or verification sheetrendered by printer 317 so that the sheet may be read by the CMI with aminimum of user effort or involvement. Reflection or transmissionmeasurements are facilitated by the transport mechanism 320 for such asheet, bearing a matrix or array of color samples, which is actuated byclick of computer mouse or, preferably, by insertion of the sheet in thetransport mechanism. The transport mechanism may be integrated with theprinter 317 in which the optical pickup component of the sensor ismounted to move in tandem with the marking head of the device andtransport of the copy may be performed by the mechanism of the printer,such as when the printer is an ink jet printer. In either case, theoptical pickup link their devices to a control unit for the CMI by fiberoptic or by electrical wire link.

The profile server 316 at Node N 312 may consist of a multiprocessor orlocally networked array of processors or high performance workstationprocessor whose performance may be enhanced by special-purpose hardware.The exact architecture (such as RISC or CISC, MIMD or SIMD) is notcritical, but needs to provide the capacity to compute quickly colortransformation mappings, gamut operations, etc., as described earlier.Any of the processors in the network may have this capability or nonemay. However, the more responsive the network is in development andmodification of Virtual Proof constituents, the more useful it can be.Disk storage or memory 321 (similar to storage 19 in FIG. 3A) representscentralized storage of current and historical constituents, which may beshared by one or more nodes on the network. The profile server furtherprovides a database which stores calibration data for rendering devicesof the network, such as color profiles (inter-device color translationfiles), or data needed to generate such profiles. The calibration dataproduced for each rendering device in the network was described earlier.

Referring to FIGS. 23-32, the calibration of video color monitors ordisplays shown in FIGS. 3A and 3B will be further described. Videodisplays play an increasingly important role in color communication.They are used to simulate three-dimensional rendering in synthetic imagecreation, as such they function as linear. 3-channel, input devices, andare also used ubiquitously in interactive image editing. Further, videodisplays can be used as soft-proofing devices, thereby providing a videoproofer. In the latter application, it is desired to portray an image ona video proofer as it will be rendered on a hard copy device. Althoughit is not usually possible to match the spatial resolution of hard copywith a soft proof, it is possible to forecast color reproduction.

Although the following describes color calibration of cathode ray tube(CRT) displays, it can also be applied to any video display technology,such as Digital Light Processing (based on Texas Instruments DigitalMicromirror Device), flat plasma panels. Liquid Crystal Display panels,etc. The foregoing technologies may be used in front or rear projectionapplications. An embodiment in which a front projection device is usedto project high resolution imagery onto production paper stock wasdescribed earlier.

Video displays are highly complicated image reproduction devices,featuring ample adjacency effects (interactions among neighboringpixels, where a neighbor may be spatially removed at some distance.)However, satisfactory results may be obtained by ignoring complexspatial effects and modeling three, separable variables simply: 1) colormixture. 2) gamma or the intensity-voltage relationship and 3) thedependence of luminous output at a pixel on where it is on the screen,independent of its interactions with the level of activity of otherpixels.

All video displays behave as light sources and those suited to accuratecolor reproduction conform to linear rules of color mixture. In otherwords, they observe, to a reasonable approximation, the principle ofsuperposition. This means that the light measured when all threechannels are driven to specified levels equals the sum of the lightmeasured when each is driven separately to the specified level. Also,the spectral emission curve (indicating light output as a function ofthe wavelength of the light) changes by a constant scale factor as thedriving voltage changes.

The last point is illustrated for green phosphor emission in FIG. 23.The emission spectrum is a property of the phosphor and is normallyinvariant during the useful life of the CRT. The graph shows theactivity in the green channel, as a function of wavelength, when drivenfull scale at digital level 255, denoted by numeral 322, compared with15 times the activity in response to digital level 64, denoted bynumeral 323. The fact that the two curves superimpose means that theydiffer by a factor which does not depend on wavelength—a linear propertyimportant for color mixture. However, the scale factors are not linearwith digital drive levels, a manifestation of non-linear gamma in thedevice. The derivation of color mixing transformation matrices capableof translating RGB device codes for a particular monitor into XYZTriStimulus Values or vice versa is described, for example, in Holub,Kearsley and Pearson, “Color Systems Calibration for Graphic Arts. II:Output Devices,” J. Imag. Technol., Vol. 14, pp. 53-60, April 1988, andin R. Berns et al., “CRT Colorimetry, Part I Theory and Practices. PartII Metrology.” Color Research and Application, Vol. 18, Part I pp.299-314 and Part II pp. 315-325, October 1993.

It has been observed that the spectral emission functions of CRTphosphors do not change over significant periods of time. This meansthat for a large class of displays, the data needed for color mixturemodeling can be measured once, preferably at the factory, and relied onfor most of the useful life of the monitor. Phosphor spectra and/orchromaticities measured at the factory may be stored in Read Only Memory(ROM) in the display, they may be written on a disk and shipped with themonitor or, preferably, they may be stored in association with themonitor's serial number in a “Cyberchrome” database at Node N 312 ofFIG. 22. It is preferable if spectral data are captured and stored. Inthe database, they can be made accessible to networks for VirtualProofing via restricted, keyed access, using typical encryption andsecurity schemes for the Internet.

The balance of Red, green and Blue channels does vary continually, oftenrequiring adjustment at least daily. RGB balance determines the whitepoint. Key variables subject to change in the R, G and B channels arethe bias and gain. The bias, or offset, determines the activity in thechannel at very low levels of input from the host computer system, whilethe gain determines the rate of increase in activity as digital driveincreases and the maximum output. Bias and gain controls usuallyinteract within a color channel, but should not interact betweenchannels if the superposition condition is to be satisfied. The gamma ofa channel is the slope of the relationship giving log light intensity,usually in units of CIE Luminance, or Y, and log digital drive suppliedby the host computer system. In order to maintain a given white balanceand overall stability of tone and color reproduction, it is necessary toregulate the bias and gain in the three channels. Consistency of gammais the main determinant of tone reproduction, while white pointstability depends on the balance of activity in the three channels.

Ambient illumination refers to light that reflects off the faceplate (orscreen) of the display and whose sources are in the surroundingenvironment. Light so reflected adds to light emitted by the display. Aviewer may not be able to distinguish the sources. At low to moderatelevels, ambient illumination affects mostly the dark point of thedisplay causing a loss of perceptible shadow detail and, possibly, achange in shadow colors. In effect, gamma (and tone reproduction) aremodified, along with gray balance in the shadows. Generally, it ispreferable to detect and alert to the presence of ambient contaminationso that users can control it than to incorporate ambient influences intothe calibration.

The foregoing several paragraphs are meant to set the stage fordiscussion of monitor calibration in the color imaging system of thepresent invention.

It is often very difficult to build a good colorimeter, as discussed inthe article by R. Holub, “Colorimetry for the Masses?” Pre magazine.May/June, 1995, for video displays. Accurate, repeatable ones areexpensive; for example, one marketed by Graseby Optronics sells foraround $6000, currently, and a fine instrument by LMT costs considerablymore. FIG. 23A shows a comparison between measurement of spectralemission of a red phosphor of EBU type by a PhotoResearch PR700Spectroradiometer 324 and a Colortron II 325. The plots make clear thatan instrument capable of resolving 2 nanometer increments can resolvethe complex peaks in the phosphor's spectrum while Colortron cannot.Colortron is a low-resolution, serial spectro as discussed earlier citedarticle “Colorimetry for the Masses?”. Calculations of TriStimulusValues and chromaticities based on the two spectra reveal significanterrors in the estimation of red chromaticity with Colortron II.

The foregoing analysis suggests that, for phosphor-based displays, atleast, continual colorimetry is not required to maintain goodcalibration. Thus, maintenance of gamma and white balance and monitoringof ambient influences is all that should be required for aphosphor-based display. Such maintenance is described later isconnection with FIG. 32.

FIG. 24 shows a configuration of color display 326 (i.e., CRT), coweland color measurement instrument, as generally shown in FIG. 3B, thatenables very accurate, inexpensive and automatic maintenance of CRTdisplay calibration. Additional features of this configuration willbecome apparent from the following discussion of FIGS. 24, 24A, 24B, 24Cand 24D.

The cowel 322 is circumferential and helps to shield the faceplate 328from stray light coming from any direction, including the desktop onwhich the display 326 may be located. Cowel 322 is black-coated on innerfaces 322 a to absorb light. The cowel 322 forms a light trap 329 forthe embedded sensor 330 coupled to the cowel. A small lamp 335 islocated beneath the lower flange 322 b of the cowel. Lamp 335illuminates the desktop without influencing the display significantly incircumstances when overall room illumination is kept low for goodviewing.

A sensor 330, often referred to herein as an electro-optical pickup, islodged in the cowel. It views approximate screen center, denoted by line331, (however other areas of the screen may be viewed) collecting andfocussing light onto the sensor. The line of sight 334 reflects off thescreen 328 and into the light trap 329 formed by the lower flange of thecowel, so that specularly reflected light will not enter the sensor. Itpermits unattended calibration, possibly during screen-saver cycles orat other times when the operating system of the computer at a node isnot scheduling activities which would compete with calibration. Becausethe sensor 330 is perched in the cowel 332, it does not require userinvolvement to be affixed to the screen, it leaves no saliva or otherresidue on the faceplate, it does not contribute to desktop clutter andit is well positioned to monitor ambient illumination influences.

FIG. 24A shows an example of the assembly of cowel and sensor for acolor monitor, wherein the circumferential cowel 401, arm member 410,and one of the brackets 404 of FIG. 24A are separately shown in FIGS.24B, 24C, and 24D, respectively. To satisfy the goal of adapting thecowel to monitors of different size and make, the circumferential cowel401 is a separable part which can be sized apropos of 17, 20, 21 or 24inch or other size monitors. It can be made of any material, butpreferably is of light weight plastic. The rear edge 402 of the cowel401 rests against that part of the plastic monitor cover which framesthe faceplate of a typical monitor. This has two advantages: First, itprovides relief to the mechanism 412 which suspends and supports thecircumferential flange 401 a of the cowel 401 because a significantcomponent of the force of support is into the front of the monitorchassis. Second, the upper flange 401 b of the cowel 401 will supportthe electro-optical pickup head at socket 403 at a correct andpredictable angle of view of the screen.

The mechanism 412 for supporting circumferential cowel 401 on themonitor includes a rack and pinion adjustment mechanism having anextending arm member 410 with a knob and gear 405 to allow adjustment oftwo brackets 404 having teeth which engage opposite sides of the gear405. The brackets 404 have ends 406 which attach to the top right andleft corners of the monitor's chassis. In other words, adjustment of theknob causes two foam feet at either end 406 of the brackets to move inand out so as to control their pressure against the sides of thechassis. In other words, the sliding bracket rests atop the monitor'schassis and is held in place by an adjustable press fit to the sides ofthe chassis. It, in turn, provides additional support to thecircumferential flange 401 a. Arm member 410 connects the brackets 404to the circumferential flange 401, such as by screws coupling arm member410 and upper flange 401(b) together. Alternatively, the knob and gear405 could be replaced by a friction-fitted tube sliding within a sleeve.Cables 407 represents wires being led from the electro-optic componentsin socket 403 back to an interface such as Universal. Serial Bus, of thecomputer 314 coupled to the monitor 311 (FIG. 22). However, it couldalso represent an alternate means of attachment, if it were madeadjustable front to back of the monitor's chassis. The arm member 410rests partially on the circumferential flange 401 at upper flange 401(b)and provides a housing for the electronics (circuitry) 408 associatedwith the electro-optic unit which plugs in at socket 403.

The depth, denoted by numeral 409 of the circumferential flange 401 a isthe distance from its outermost edge along a perpendicular to the pointat which it rests against the monitor's chassis. Due to contouring ofthe chassis or the cowel (in the interests of a good fit to the chassisor of aesthetics) the depth may vary around the circumference. We havechosen, for example, a depth of 8 inches, on average, to effect a tradeoff between two factors: the degree of shielding of the viewing area ofthe screen from environmental stray light (ambient), and thedesirability of enabling more than one user an unimpeded view of thescreen at one time. At 8 inches, it should be possible for 2 or 3 peopleseated in front to see the full screen and for 2 or 3 people standingbehind them also to see. However, the particulars cited here are notmeant to limit the generality of the invention.

Many users like to conduct soft proofing under circumstances in whichthe hard copy proof may be available for comparison to the soft proofvisualized on the monitor. Under said circumstances, it is important tocontrol the viewing conditions of the hard copy. A viewing hood such asthe SoftView™ made by Graphic Technologies, Inc. is often used for thispurpose. However, it is also important to insure that the viewing hooddoesn't contribute to stray light and that it is positioned tofacilitate soft- and hard-proof comparisons.

Accordingly, the viewing box (or reflection viewer or hood) 501 shown inFIG. 25 may optionally be provided at Node A 310 (FIG. 22) which isintegrated with the cowel 401. FIG. 25 shows an arrangement in which thevideo display is raised up on a pedestal with viewing box positionedbelow, so that the top side of the box is contiguous with the bottomflange 401 c of the cowel. One side of the box has an opening 605opposite the back wall 608 of the box upon which media is locatable.Other arrangements are possible, for example, some workspacearrangements may call for situating viewing hood and monitorside-by-side. i.e., integrated to the right or left side of the cowel,and oriented at an angle so as to optimize viewing at relatively shortrange.

FIG. 26 is a scale drawing, showing the configuration of light sources601, lenses 602 and reflectors 603 in the viewing box 501, whose sizeshould be adequate to accommodate at least a two-page spread inpublication terms (11×17 inches.) At top and bottom (or left and right)are ballasts 604 which can accept one or two (depending on requiredillumination levels) Daylight 5000-simulating fluorescent lamps. D5000is mentioned merely because it is a standard illuminant in certainindustries; in other industries, the choice might be different.Approximately pear-shaped plastic lenses serve to diffuse and dispersethe light. The lenses should be frosted or waffled (rippledlenticularly) so as to enhance diffusion of light and should not greatlychange the color temperature of the fluorescent light.

Any direct sighting of the lamps/diffusers by the viewer 501 (in normaluse) is prevented by reflectors 603 which are aluminized or chromed onthe side facing inward toward the copy so as to reflect and furtherdisperse the light. The configuration depicted has been shown to resultin very uniform, diffuse illumination over the area of at least a twopage spread. Angles 605 and 606 are approximately 14° and 34°,respectively. At eye 607 is depicted on a line of sight which shouldextend at least 25 inches back from the rear viewing surface 608 of theviewing box. The single lines 609 and double lines 610 indicate theextent of the illuminated back wall of the viewer which can be seen fromdistances of 40 inches and 30 inches, respectively.

The above discussed assembly is preferably equipped with a dimmer. This,along with the choice of number of fluorescent tubes, allows criticalcontrol of the brightness levels within the viewing box. Brightnessshould be made as similar to that of a full-field white on the videodisplay as possible. An additional option for the viewing box 501 is theinstallation, in the back wall 608, of a light box able to accommodate8×10 or smaller color transparencies, as is provided in otherconventional viewing hoods used in industry.

As explained previously, the primary chromaticities of phosphor-baseddisplays are very constant over time. Therefore, the sensor coupled tocowel 332 (FIG. 24) or 401 (FIG. 24A) should be a simple, calibratedluminance meter, called thereinafter lumeter, sufficient to maintainwhite balance and gamma and to detect significant increases in ambientillumination over the norm.

However, display technologies which rely on light sources, such as LCDor DMD as defined earlier, may experience drift in primarychromaticities as the external light source ages. It is common for frontor rear projection displays based on liquid crystals or digitalmicromirrors to employ metal halide lamps or xenon arcs. The spectraldistribution of the former certainly changes with time and that of thelatter may under some circumstances. In these cases, adequate control ofcolor balance requires ongoing colorimetry, such as provided by aspectral based CMI. Even with CRTs, there are occasions in whichadequate calibration or re-calibration may involve a color-capabledevice. For instance, it may be desirable to retrofit a CRT which wasnot calibrated at the factory with the invention in order to confer itsbenefits on a user. Accordingly, several types of electro-optic modulesare described below, from a Lumeter to a spectral device.

Referring to FIG. 27, a cross-section of part of the sensor integratedwith the upper flange 401 b of the cowel in socket 403 (FIG. 24A) isshown having a housing 701 including optics. On the assumption that thecenter of the outer edge of the upper flange is 7.5 to 8 inches from thechassis and that the flat surface of the flange is perpendicular to thefaceplate, the center of the hole bored for the lens tube is 6 inchesout. The bore is cocked at an angle of 53° to the flat surface of theflange so as to look at the approximate center of a nominal 21 inchscreen. For example, the length of socket 403 may be 2 inches, and 1inch in diameter.

The lens 702, shown in cross section in the tube, is a commercial gradecondenser, plano-convex with diameter of about 20 mm and focal length ofabout 25.4 mm. It actually forms an image of what is at screen centerabout 36 mm behind the lens, presumably due to the diffuse and divergentnature of the light source. The inner surface of housing 701 should bevery light absorbent. It should be painted flat black, or, preferably,outfitted with a conical baffle 704 with porous black inner surface. Theabove numerical values of components in FIG. 27 are exemplary, and othervalues may be used. In particular, a 53° angle is not quite as desirableas 45°, but the important consideration is that the line of sightreflected off the center of the faceplate intersect the black trapformed by the bottom flange of the cowel, as shown in FIG. 23. The blacktrap shields the sensor from light rays specularly reflected off thefaceplate of the monitor.

FIG. 27 also shows an inner tube 705 which slides into the outer lenstube on tracks 707. It is used to bring either a photodiode array or afiber optic 706 up to the focal plane behind the lens. The photodiodearray is employed in the simple lumeter while the fiber optic pickup isused with a spectrograph. In the lumeter, it is desirable not toposition the sensor exactly in the focal plane so as to induce a littleblurring over the sensor. In the case of the spectrograph, the fiberoptic should be chosen to have an acceptance angle which is ascompatible as possible with lens 702 and with the spectrograph to whichit is coupled to.

Separate tubes may be used for the lumeter and the spectrograph. Theformer could employ a non-achromatic plastic lens whereas the latterbenefits from a glass achromat. The two kinds of tube should mate withthe flange interchangeably.

Referring to FIG. 28, is a block diagram of the lumeter is shown withcontrol circuitry. The lumeter includes two parts which may be combinedor separate from each other, an electro-optic pickup head 801 and thecontrol and interface electronics circuity (controller) 802. Theelectro-optic pickup head 801 includes a lens 702 which converges lightonto sensor 804 through spectrally selective filter 803 which must, atthe least, attenuate Infrared radiation greatly. Another sensor 805 isan optional sensor identical to sensor 804 except that it is thoroughlylight-baffled (i.e., protected from receiving any light).

Sensor 804 may be a photodiode array incorporated in a TSL 230, theprogrammable member of Texas Instruments' Light-to-Frequency converter(LTFC) family. The device integrates photosensors with an amplificationstage and a current to frequency output stage which interfaces directlyto a counter on a microcontroller. The lumeter described is verydesirable because of the high levels of sensitivity, repeatability andlow parts count.

Because its output is a pulse train, the TSL 23X device can be coupledto the control electronics 802 by a lead of up to several feet.Therefore, the device may be fitted into the inner tube with amplebaffling and positioned near focal plane. Wires are lead back to thecircuit board containing the control electronics. In this way, theelectro-optical pickup head 801 can be very compact. As stated earlier,the control electronics 802 may be located in a housing or cavity 408(FIG. 24A) in arm member 410 connecting the brackets 404 to thecircumferential flange. Such control electronics may also be located aseparate circuit enclosure.

Circuitry 802 consists of the control and interface electronics,including a microcontroller 807 having an on-board hardware counterwhose overflows are counted in software. This enables long lightintegration times. A multiplexer 806 of circuitry 802 selects betweenthe two sensors 804 and 805 when both are available, while timing iscontrolled by a clock oscillator 808. A level shifter 809 provides aninterface for adjusting the output of the microcontroller for RS232,RS422, or other communication protocol, such interface may be a USB(Universal Serial Bus) interface.

The lumeter operates by cumulating pulses from the '23XLight-to-Frequency Converter (LTFC) so as to perform ND conversion byintegration. It is best to use some means to ascertain the refreshfrequency of the display and to set the integration time to be anintegral multiple of the refresh period. One means is to read back therefresh frequency from the processor in the display. Another is tomeasure it with the lumeter, taking advantage of Fast Fourier Transformalgorithms and techniques, such as described, for example, inapplication notes published by Texas Instruments for the TSL 23X.However, it is also acceptable to set an integration time such thaterrors due to incomplete refresh cycles are small. An integration timeof 5 seconds satisfies this criterion for a 75 Hz refresh rate.

FIG. 29 shows a basic command set used by the host computer tocommunicate with the lumeter. For example, the series of ascii strings“<E1>” “<P0>” “<D0>” “<S2>” “<T5F5>” is used to initialize the lumeterfor data collection, where the ascii strings are enclosed in quotationmarks. The effects are to turn on echoing from the lumeter back to thehost, to power up the sensor so that it stays “awake” once a sensitivitycommand is issued, to program frequency division to a factor of 1, toset sensitivity to 100, i.e., use all available photosensors, and to setthe integration time to 5 seconds However, in production, it ispreferable for most applications to use a non-programmable version ofthe LTFC such as the TSL 235.

Intensity/Voltage data collected with a reference spectroradiometer (aPhotoResearch PR700) and with a lumeter are assembled in Table I below.These are the data needed to calculate gamma. Data are presented forseveral conditions which test the generality and robustness of thedesign. FIG. 30 exemplifies the summary data analysis. The actual points1002 come from the fourth and fifth columns of the second data set,namely full screen green at 23° C. ambient temperature. The values ofgamma quoted in what follows are the slopes of straight lines such as1001.

TABLE I Measurement of Intensity/Voltage Characteristic for SeveralConditions Digital Lumeter PR700 Log Log Log Code Counts LuminanceDigital Lumeter Luminance Measured for 6″ by 6″ square centered inscreen. green channel 15 10 0.0398 1.1761 1 −1.3997 31 65 0.3408 1.49141.8129 −0.4675 64 350 2.191 1.8062 2.5441 0.3406 128 1349 9.552 2.10723.13 0.9801 255(max) 4503 33.73 2.4065 3.6535 1.528 Measured from fullscreen, green channel, room temperature of 23 C. 15 18 0.0397 1.17611.2553 −1.4008 31 132 0.3532 1.4914 2.1206 −0.452 64 699 2.185 1.80622.8445 0.3395 128 2627 9.401 2.1072 3.4195 0.9732 255(max) 8537 32.632.4065 3.9313 1.5136 Measured from full screen, green channel, roomtemperature of 30 C. 15 23 0.0495 1.1761 1.3617 −1.3057 31 144 0.38831.4914 2.1584 −0.4108 64 732 2.311 1.8062 2.8645 0.3638 128 2696 9.6422.1072 3.4307 0.9842 255(max) 8656 33.05 2.4065 3.9373 1.5192

Gammas calculated in the manner of FIG. 30 for the various conditionsare:

-   -   a) for the Lumeter, 2.15, 2.17 and 2.08 for the 6×6 square at 23        C, full field at 23 C and full field at 30 C, and    -   b) for the PR700, 2.37, 2.36 and 2.29 for the same conditions,        respectively.        Lumeter-derived gammas are a fixed percentage of PR700 values,        indicating a robust calibration strategy and good immunity from        field size. Both instruments appear to have good temperature        compensation; however, there is about a 5% loss of gamma at the        higher temperature. This is due to increase in dark current at        the higher temperature. For applications in which higher        temperatures may be encountered and accuracy to better than 1%        is desired, a second, light-baffled sensor (805 in FIG. 28)        should be added. Adding a temperature-sensing detector is less        expensive than shuttering the system, however a shutter for        sensor 804 could alternatively be used. Because sensor 805 is        baffled, its readings indicate the dark current which can be        calibrated against temperature. Background temperature readings        can be taken with long integration cycles to insure accuracy        when the instrument is not being used for video display        calibration.

The gammas discussed above are not identical to those of a trueluminance meter because the lumeter does not have the photopicsensitivity function. However, it is preferable to save the spectralemission characteristics of each phosphor channel, as noted earlier, andto measure the spectral sensitivity function of each lumeter as part ofthe calibration process. In that way, it will be possible to calculateexactly the relations between true luminosity data and lumeter responsesfor any phosphor set and any actual lumeter. This is done by convolving(with a shift parameter of zero) the spectral emission function with thehuman sensitivity function and with the lumeter's spectral sensitivity.In this we are helped by the linear scaling of spectral emission withdrive voltage. However, if spectral data on phosphors is not available,very good calibrations will be possible based upon generic data.

The lumeter provides reproducible results from day to day, and is atleast as stable as typical reference light sources, given a suitablechoice of spectral filtering and a stable lens material. Therefore, theonly aspect of self-calibration which is of concern is the monitoring oftemperature-dependent dark signal. This is accommodated in the inventionso that the lumeter is self-calibrating, automatic and non-contact, aswill be described later below.

Referring to FIG. 31, the high level operation of the system forsoft-proofing is shown. Users want to be able to edit or retouch imagedata while seeing the images they are preparing on the video display asthey will appear on hard copy to the greatest degree possible, such asdescribed, for example, in the article by Holub, et. al., J. Imag.Technol., Vol. 14, p. 53. Automatic device calibration at a singleinstallation or coordinated among multiple nodes is provided by theabove discussed lumeter and cowel assembly.

Data generally come from one of two sources in a soft-proofingapplication. CMYK data 1101, or as 3-dimensional color data 1102,usually in a device-independent coordinate system such as CIELAB orcalibrated RGB. The first step is to get the data into the form of 3-Dcolor′ data 1103, where color′ means that all the colors are printable.Since CMYK data are printable by definition, they need only betranslated to suitable 3-D coordinates, as described earlier inconnection with FIG. 4A. To effect the conversion for data that startout in a device-independent notation, they must be processed through agamut operator. The latter is based upon gamut descriptors derived frommodels of the CMYK and display devices which was also described earlier.

Finally, the 3-D color′ data must be translated into display-specificRGB signals. The result is accurate color output. Literal colorimetricreproduction on the video display may not be possible, such as describedin the article Holub, et al., op. cit.) The color translator 1104 mayinvolve processing beyond that needed to convert device-independentcolor′ data into calibrated RGB for the particular display. However, thelatter conversion is a critical part of the translator, and the concernin this discussion is to show how it is formed or updated automatically.

Referring to FIG. 32, a flowchart of the procedures for using thelumeter to set up a video display for soft-proofing and for remoteproofing. It assumes that the color mixture component of the devicecharacterization problem has been dealt with, such as by one of themethods taught in articles cited earlier. For simplicity, assume thatthe chromaticity data derived from factory measurements of the display'sR, G and B channels are combined with data about the desired white pointto form a 3×3 matrix. The color translator 1104 then consists of a stagewhich converts 3-D color′ data into XYZ TriStimulus Values, ifnecessary, which are then converted into linear RGB by the matrix.Resulting R. G and B values are processed through Tone ReproductionCurves (“TRCs” in the parlance of the International Color Consortium'sProfile Format Specification.)

Whatever the sequence of processing steps implicit in a colortranslator, they will only be effective if the parameters of processingcorrespond to the actual state of the device. In other words, thechromaticities used to compute the matrix must be those of the device,the three channels must be balanced to the correct white point and theTRCs must compensate for the non-linear gamma of the device. Insuringthat the device is in a targeted state called calibration is the objectof FIG. 32.

State 1201 shows the Lumeter in an initial state of periodic dark signalreading. This state may be interrupted by initiative of an operator oron cue from the operating system as described earlier. Anautocalibration cycle begins by darkening the screen 1202 and measuringthe sum of light emitted by the screen (the dark emittance) andreflected off the screen by environmental sources not baffled by thecowel. Call the sum dark light.

One of the shareable components of the Virtual Proof is a default ornegotiated tolerance for dark light. If this is exceeded, credible softproofing at a given node and collaborative proofing between remote nodesmay not be possible. The device driver for the lumeter should flag theevent 1203. During a prolonged period of system inactivity, it may raisethe flag repeatedly; when an operator returns to use the system,however, he/she should be advised that an unattended calibration cyclewas not possible. There are two main ways of recovery: a) restore darklight to an acceptable level (operator action), such as adjusting theamount of ambient light reflected from the screen, or b) use a morecomplicated model of color mixture, if possible, which includes theeffects of the dark light.

It should be noted that the lumeter cannot discern the color of darklight. The latter may be important in some applications, either becauseof fairly high levels of dark light or due to a need for very criticalcolor judgments. When measurement of the color of dark light isnecessary, replace the lumeter with a spectrograph, as described latterin connection with FIG. 33.

Once the question of dark light is resolved, neutral balance 1204 isestablished. The aim white point is used in a computation of therelative activity levels needed in R. G and B channels to achieve thetarget at highlight and in the shadow. First the gains of the amplifierswithin the color display which control electron density as a function ofexternal drive are adjusted in each channel until a criterion balance isrealized. This establishes highlight white. Next, the biases of thosesame amplifiers is adjusted until the appropriate balance is realized inthe shadows. This is likely to upset the highlight balance. Therefore,several iterations may be needed to balance both highlight and shadow.

Under the simplest model of color mixture, the computation of relativeactivity levels proceeds as follows: From the factory-suppliedcalibration data, we have a 3×3 matrix of chromaticities, a column ofx,y,z for Red, a column for Green and a column for Blue. The inverse ofthe foregoing matrix is dotted (a matrix inner product is formed) withthe vector of white chromaticity values to yield a vector of weights.When the first weight is multiplied by each entry in the first column ofthe original matrix, the second weight by each entry in the secondcolumn, and so on, an RGB to XYZ matrix results. In particular, thesecond row of said matrix holds the Y TSV, or luminance value, thatshould prevail in each channel at the aim white point. The lumeter canbe calibrated as described earlier, in connection with FIG. 30, tomeasure the luminance values so that the system software can decide whatchanges of gain or bias are needed. The foregoing description is meantonly to clarify how the lumeter is used, not to restrict the scope ofthe invention to this model of color mixture.

Next, the gamma is measured in each channel 1205 by commanding a seriesof digital levels and measuring the result. If these need to have veryspecific values, but do not, then it is necessary to re-adjust gain andbias and re-iterate over neutral balance. If the gammas are critical,then the gammas should be adjusted to be close to the target valuebefore neutral balancing. If the gammas merely need to be within a givenrange, then the TRCs in the profile can be adjusted to reflect the realstate of the device once the values are within the desired range. Oncompletion, the profile is updated 1206 and the Lumeter can return tosedentary (standby) mode 1201.

The adjustment of the TRC is accomplished as follows, referring to FIG.30 If the aim value of output for a given command (this is the addressin the TRC lookup table or the independent variable) is 1210, then entertables of aim and measured values as a function of digital command levelto find that level which produces the aim value 1211 among the measureddata. That level becomes the entry or dependent value in the TRC LUT. Inthe interests of avoiding quantization artifacts, it is preferable touse precision greater than that afforded by 8-bit-in/8-bit-out LUTs.

For most video display technologies, with the possible exception ofthose which rely on microfilters to provide wavelength selection (e.g.,LCDs,) the problem of color differences in different regions of anominally uniform color field can be separated from the aspects ofdevice modeling that involve color mixture and gamma. For example,methods for flattening the screen are described in U.S. Pat. No.5,510,851. The sensor and cowel assembly of FIGS. 23 and 24 can providean automatic means of measuring non-uniformity of the display. Aninexpensive, black & white, solid state camera, fitted with a wide fieldlens, may be located and centered in a door assembly in cowel 401 whichcan covers the opening 411 in the cowel 401 (FIG. 24B.) It is preferableto eliminate the effects of environmental light on spatial homogeneitymeasurements. The camera should view slightly defocussed screen in theinterest of anti-aliasing, one color channel at a time. The digitizedimages are then analyzed for non-uniformities and corrections computedand applied, such as described in the above cited patent. The distortionfunction of the wide-field lens should be measured and factored into thecalculation of correction factors, as should differential sensitivityacross the sensor array.

The following will describe color measure instruments utilizingspectrographs compatible with high resolution measurement of phosphoremissions from color monitors. i.e., CRTs to provide non-contactmeasurement, self-calibration, and push-button operation. The termpush-button operation refers to the capability of a user automaticallyinitiating color calibration, or that such color calibration isinitiated automatically by a computer coupled to the monitor. As statedearlier in connection with FIG. 27, the sensor of the lumeter can beseparate from the control and interface electronics. Sensor of thelumeter could also be a fiber optic pickup, with collecting optics, thatcould be coupled to a spectrograph. Therefore, the arrangement forspectral monitor calibration is non-contact and automatic once thesensor is fitted in the circumferential cowel.

Self-calibration of a spectral instrument involves insuring thatreadings are associated with the correct wavelengths and have the properrelative or absolute amplitudes. Unless damaged electrically or byradiation, solid state sensors tend to be very stable over time, more sothan most lamps. For this reason, reference detectors are conventionallyused against which standard lamps can be calibrated. Temporal variationsin a source are best dealt with, in reflection or transmissionmeasurements, by a dual beam technique, wherein the light source isreflected (or transmitted) off known and unknown objects eithersimultaneously or successively, depending on the temporal stability ofthe source. The spectral properties of the unknown object are inferredfrom the ratio of its spectrum with that of the known object.

A pulsed xenon source has many advantages for reflection andtransmission spectroscopy. It emits strongly in the short visible waveswhere silicon detectors are less sensitive. It has a very stereotypicalarray of impulsive spectral lines across the visible spectrum which arevery useful for wavelength calibration of spectral sensors. However,output is not stable from flash to flash making them best suited to dualbeam designs.

Referring to FIG. 33, a block diagram of a color measurement instrumentfor calibrating a color monitor is shown utilizing a spectrograph.Source 1301 shows a pulsed xenon lamp tethered over a short distance toits power supply 1302. Source 1310 may be Xenon modules, such asmanufactured by EG&G of Salem. Massachusetts. Two fiber optic taps aretaken, tap 1303 to the reference input of the dual beam spectroscope1307 and tap 1304 to illuminate the reflection sample. Tap 1303 isbifurcated so that light is also taken to an assembly 1305 consisting ofat least two Light-to-Frequency Converters (LTFCs) whose purpose will bedescribed presently. Pickup 1306 is a fiber optic pickup which canaccept light reflected or transmitted from a sample. Alternatively, itcan be placed in the sensor assembly shown in FIG. 27, for use inspectral monitor calibration. Although the fiber optics are employed forflexibility, they must not be flexed during use and the components ofthe assembly that are linked by fibers must bear a fixed relationship toone another. The term fiber optic tap refers to one or more fiber opticcables.

The LTFCs in assembly 1305 serve two purposes. By monitoring theiroutput in the dark, the temperature of the assembly can be estimated aswe have seen earlier. However, they are also used as referencedetectors. Each receives light through a different, spectrally-selectivefilter. For example, two LTFC's may be used, one tuned to a particularpeak in the short wave region of xenon output and the other tuned to apeak in a longer wave region. In this way, they serve as a check on theconsistency of the sensitivity of the reference sensor in thespectrograph at a given temperature. They also aid in estimating thedark current in the reference line camera. In summary, they contributeto checking amplitude calibration of the spectrograph, which may becontrolled by the control electronics of the spectrograph. As notedearlier, the pulsed xenon spectrum provides very predictably placedpeaks for use in wavelength autocalibration. The two aspects ofcalibration discussed here need not be done simultaneously in the caseof monitor measurements; it is enough that they have been done recently.Thus, the signals from the LTFC sensors can be used to provideinformation for automatically checking the calibration of thespectrograph.

Referring back to FIG. 3C, the color measurement instrument of FIG. 33may also be used for reflection color measurement from a hard copysample, or media. The fiber optic probes 36 disposed over the papersample were given a 45/90 geometry. This geometry works well, especiallywhen the illuminating fiber oriented at 45° has a tip of oval shape suchthat the light spot formed is approximately circular with diameter 3-5mm and uniformly illuminated. However, the foregoing geometry may not besuited to tight quarters. Two other configurations also produce verygood results, even when the illuminating and detecting fiber probes havenearly the same orientation (typically near vertical) with respect tothe copy. One configuration includes a polarizer placed over the tip ofthe illuminator fiber optic and a polarizer that is crossed, withrespect to the first, is placed over the detector fiber optic. Specularpickup is severely attenuated, provided that crossing is almost perfect.Alternatively, a diffuser may be placed over the illuminator fiberoptic; in this case, the end of the fiber should be several millimetersbehind the diffuser. This is easier to setup than the polarizers, butmay produce a larger illuminated spot than is desired in somecircumstances. In each of the above configurations, all measurements arenon-contact. Therefore, they do not disturb the copy sample beingmeasured. The configuration we describe does not require significantbaffling of extraneous light due to the very considerable (in a briefinterval) output of pulsed xenon.

The preferred spectrographic design for simultaneous, dual beam work isone described by J. T. Brownrigg (“Design and Performance of a MiniatureDual Beam Diode-Array Spectrometer,” Spectroscopy. Vol. 10, pp. 39-44.November/December '95) and manufactured by American Holographic(Fitchburg. Massachusetts) as the MD-5. For applications in which thespectrograph and related modules are embedded in a printer, such as anink jet plotter with a single, mobile head (rather than a web design,)the design discussed in the article has satisfactory spectralresolution. Other spectrographs providing comparable performance mayalternatively be used.

However, for applications in which the unit (the color measurementinstrument of FIG. 33) is installed in a pen-plotter-like assembly formeasurements of specialized calibration forms and occasionally removedfor monitor duty, a modified design is needed to achieve adequatespectral resolution. In it, the optical bench is lengthened and agrating with superior dispersion and focussing properties is used andthe system must be aligned to optimize focus in the long wave region ofthe spectrum in which the paper/copy is transported throughautomatically while the pen scans the paper perpendicularly to thedirection of the advance.

The pen-plotter assembly transports the paper or copy automatically whena pressure switch senses its presence. The fiber optics described abovereplace the pen and scans the copy perpendicularly to the direction ofpaper advance. Hence, we have a non-contact measurement by an instrumentthat is fully self-calibrating and provides push-button simplicity. Thisconfiguration could easily subserve the color measurements needed insupport of soft-proofing as discussed in connection with FIG. 31 andearlier in this description. If one or more instruments of the kinddescribed were available in a network for virtual proofing, they mightbe shared for hard-copy or, when needed, for monitor calibration.

A less sophisticated calibrator may be used for inexpensive printdevices. For “low end” devices, the strategy of choice is to make stockprofiles available for identified lots of inks or toners, through theCyberchrome Service (FIG. 22) and then to use the calibrator to makesure that the devices conform as much as possible to a setup consistentwith the assumptions used in making the profile. This would entail, at aminimum, ensuring that the Tone Reproduction Curves (TRC) for eachchannel in the printer conformed to specification.

Optionally, it would ensure that the image area of the device is flat.i.e., an uniform image reproduces uniformly across and down the page.This would require techniques analogous to those described in the artfor flattening video displays, and referred to earlier in thisdescription. In essence, at each point on the page, the TRCs areadjusted to match the least commonly achievable TRC and some sort ofinterpolation is employed to feather the corrections applied tocontiguous blocks on the page for which TRC corrections were computed. Auseful instrument for measuring the non-uniformities and initial TRCsfor the different color channels is a monochrome scanner such as thePaperPort, marketed by Visioneer of Palo Alto, Calif.

This is an advance over Bonino's patent (U.S. Pat. No. 5,309,257) in tworespects: a) distribution of colorant-lot-specific profiles and b)flattening the page. Each of these have considerable impact on the colorreproduction of any device, especially less expensive ones.

Referring to FIG. 34, a block diagram of a color measurement instrumentutilizing a concentric spectrograph is shown for providing spectralimaging colorimeter. Such a color measurement instrument is preferablyused for page printers, such as a xerographic press. Concentricspectrographs are described by L. Mertz, Applied Optics. Vol. 16, pp.3122-3124. December 1977, and are manufactured by American Holographic(Fitchburg. Massachusetts) and Instruments SA (Edison, N.J.) Source 1401is an illumination source. By virtue of the spectral analysis, theinterpretation of object colors can be independent of the source,opening the possibility of simulating the appearance of a product, for acustomer, under various viewing conditions.

Reflector 1402 is a conventional reflector and pickup 1403 a fiber opticpickup which collects light from the reference reflector for conveyanceto the spectrograph. Light from the reference reflector is shown on thesample object. Light reflected from the sample object is formed on a row(streak) of pixels indicated by fibers 1407. Fibers 1407 can be a row offiber optic bundles, or samples (pixels) of the image formed by a lensinstead (A.) Collection of reference light by 1403 enables a dual beamoperation in which the influence of the source illuminant can beextracted from each pixel. Fiber 1404 is a fiber which sees a black trapand is used to generate reference dark current on the array. Fiber 1405is a fiber (or generally, an input) which transmits light from a sourceused for wavelength calibration, such as source providing light of knownwavelength(s).

Surface 1408 is the reflecting surface of a concave mirror whose centerof curvature is shared with convex holographic grating surface 1409;this is a simplified depiction of a concentric spectrograph (B.) Rays1406 _(ab) and 1406 _(bc) show the flow of image rays to imaged pixelsat the entrance slit, of the spectrograph and hence to the sensor array1410, on which one dimension encodes wavelength and the other distancein space along the imaged streak (C.) In other words, the concentricspectrograph outputs spectra spatially related to points along the lineof light provided to the spectrograph. Thus, the wavelengths of eachspectrum related to light received from each of fibers 1403, 1404, and1405 can provide calibration reference information for automaticallycalibrating the spectrograph.

The spatial resolution of a practical implementation is such thatanti-aliasing filters, described earlier, are required. Conventionalcompression algorithms must be applied to the spectral data to make itsufficiently compact for storage. Where data need to be captured at highspatial resolution, a separate line camera with approximate photopicsensitivity should be resolution encoding of images as practiced in anumber of commercial imaging architectures such as Kodak's PhotoCD andthe FlashPix. When used as a digital camera, the sensor arrays must betranslated across the plane of the subject. When applied to on-pressmeasurement of color across the web, the arrays are stationary and theweb moves. Thus, the transport mechanism 320 (FIG. 22) for a physicalcopy and associated light-collecting optics should constitute a moduledistinct from the module which attaches to video display and from themodule containing sensor(s) and control electronics. Light-collectingmodules may be connected to the control module by fiber optic links.

As a spectral, imaging colorimeter, the capture device will require thespeed and image quality insured by a concentric optical design alongwith anti-aliasing software or optical design and with facilities(hardware and/or software) for compressing and manipulating spectraldata and for hierarchical, multi-resolution image storage compatiblewith conventional image data protocol, such as Flash Pix.

Although the above description relates to the printing and publishingindustry, it is also applicable to other industries, such as textileprinting. Further, in packaging and related industries, more than fourcolorants may be used in circumstances in which no more than fourcolorants overlap within a given region of the page. The system canaccommodate this by applying the herein methods to separate regions ofthe page.

From the foregoing description, it will be apparent that there has beenprovided a system, method, and apparatus for distributing andcontrolling color reproduction at multiple sites. Variations andmodifications in the herein described systems in accordance with theinvention will undoubtedly suggest themselves to those skilled in theart. Accordingly, the foregoing description should be taken asillustrative and not in a limiting sense.

1-41. (canceled)
 42. A system for capturing a two-dimensional imagecomprising: optics for collecting light reflected from, transmittedthrough or emitted by a surface or a medium; an optical element forseparating said light collected by said optics into spectral components;a module comprising an array of sensor elements which receives from saidoptical element said spectral components of said light and convertselectrical signals of said sensor elements into spectral datarepresentative of points along said image, wherein said module operatesin calibration to a reference, said reference being traceable to astandard; one or more processors to control the conversion of saidelectrical signals and to communicate through an interface to anexternal system; and software executable by one or both of said one ormore processors or said external system comprising: a program forprocessing said spectral data to produce a spectral distribution oflight for each of said points along said image; a program for computing,based upon said spectral distribution for each of said points, values ofone or more variables that provide a representation of said spectraldistribution, wherein said one or more variables are utilized at leastin rendering a visual representation of said image; a program thatenables communication of at least said values of said one or morevariables through said interface to said external system; and a programfor storing said at least said values of said one or more variables in adatabase along with information having at least time of capture of saidimage.
 43. The system according to claim 42 wherein said reference forcalibration is incorporated in said system.
 44. The system according toclaim 42 wherein at least one of at least said array or said surface ormedium move with respect to the other.
 45. The system according to claim42 wherein said external system comprises a computer system havingmemory for storing said database, in which information furtheridentifies a source of said image.
 46. The system according to claim 42wherein said one or more variables representative of a point on saidimage have values comparable to desired values for a plurality of saidpoints to provide information utilized for quality control.
 47. Thesystem according to claim 46 wherein said software further comprises aprogram that prepares one or more histograms comprising a plurality ofsaid values for comparison to one or more corresponding histogramscomprising desired values.
 48. The system according to claim 47 whereinsaid software further comprises a program that computes differencesbetween said one or more histograms and said one or more correspondinghistograms and compares said differences to an error criterion.
 49. Thesystem according to claim 48 wherein information related to saiddifferences is utilized for process control in a production system. 50.The system according to claim 46 wherein said rendering is responsive touser-modifiable tonal transfer curves for one or more channels of acolor rendering device.
 51. The system according to claim 42 whereinsaid program for computing said values of said one or more variablesconvolves said spectral distribution with one or more spectral functionscorresponding, respectively, with each of said one or more variables.52. The system according to claim 51 wherein said one or more spectralfunctions are related to human color sensitivity.
 53. The systemaccording to claim 52 wherein said visual representation of said imageis prepared with the aid of at least one color profile, said colorprofile comprising one or more color transformations and fieldsidentifying said one or more color transformations in which said one ormore color transformations comprise at least a conversion of said valuesof said one or more variables into device dependent coordinates for saidrendering.
 54. The system according to claim 42 further comprisingreferences enabling estimation of dark current of the array andverification of wavelength calibration of said module.
 55. The systemaccording to claim 52 wherein said measurement module further comprisesan input that enables measurement of the spectral distribution of lightincident upon said surface or medium, said measurement useful forrepresentation of the colors of an image of said surface or medium underdifferent conditions of illumination.
 56. A system for capturing atwo-dimensional image comprising: optics for collecting light reflectedfrom, transmitted through or emitted by a surface or a medium; ameasurement module comprising a two-dimensional array of sensor elementsand a concentric optical element having a grating that provides to saidarray of sensor elements a spectral dispersion of said light collectedby said optics, wherein said array of sensor elements records a spectraldistribution of said light collected by said optics at each point alonga line through an image of said surface or medium, wherein movement ofsaid at least one of at least said array of sensor elements, or saidsurface or medium, enables capture of a plurality of ones of said line;said system operates in calibration to a reference traceable to astandard; one or more processors to control said measurement module andto provide an interface to an external system; and software executableby one or both of said one or more processors or said external systemcomprising: a program for computing, based upon each said spectraldistribution, values of one or more variables that provide arepresentation of said spectral distribution, wherein said one or morevariables are utilized at least in rendering a visual representation ofsaid image; a program that enables communication of at least said valuesof said one or more variables through said interface to said externalsystem; and a program for storing said at least said values of said oneor more variables along with information having at least a time ofcapture of said image.
 57. The system according to claim 56 wherein saidreference for calibration is incorporated in said system.
 58. The systemaccording to claim 56 wherein said external system comprises a computersystem having memory for storing said one or more variables and the timeof capture of said image in a database along with informationidentifying a source of said one or more variables.
 59. The systemaccording to claim 56 wherein said one or more variables representativeof a point on said image have values comparable to desired values for aplurality of said points to provide information utilized for qualitycontrol.
 60. The system according to claim 59 wherein said softwarefurther comprises a program that prepares one or more histogramscomprising a plurality of said values associated with said one or morevariables for comparison to one or more corresponding histogramscomprising desired values.
 61. The system according to claim 60 whereinsaid software further comprises a program that computes differencesbetween said one or more histograms and said one or more correspondinghistograms and compares said differences to an error criterion.
 62. Thesystem according to claim 61 wherein information related to saiddifferences is utilized for process control in a production system. 63.The system according to claim 59 wherein said rendering is responsive touser-modifiable tonal transfer curves for one or more channels of acolor rendering device.
 64. The system according to claim 56 whereinsaid program for computing said values of said one or more variablesconvolves said spectral distribution with one or more spectral functionscorresponding, respectively, with each of said one or more variables.65. The system according to claim 64 wherein said one or more spectralfunctions are related to human color sensitivity.
 66. The systemaccording to claim 65 wherein said visual representation of said imageis prepared with the aid of at least one color profile, said colorprofile comprising one or more color transformations and fieldsidentifying said one or more color transformations in which said one ormore color transformations comprise at least a conversion of said valuesof said one or more variables into device dependent coordinates for saidrendering.
 67. The system according to claim 56 further comprisingreferences enabling estimation of dark current of the sensor array andverification of wavelength calibration of said measurement module. 68.The system according to claim 65 wherein said measurement module furthercomprises an input that enables measurement of the spectral distributionof light incident upon said surface or medium, said measurement usefulfor representation of the colors of an image of said surface or mediumunder different conditions of illumination.
 69. The system according toclaim 56 further comprising a line camera, operable in concert with saidtwo-dimensional array of sensor elements, in which said line cameraprovides higher spatial resolution of image capture than said twodimensional array of sensor elements, wherein images captured by saidsystem are stored in a hierarchical multi-resolution format.
 70. Thesystem according to claim 56 further comprising variable focus andcompensation for aliasing in signal processing.
 71. The system accordingto claim 51 comprises an unitary colorimeter adapted to scan rasters ofimage data.